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Well, it's not exactly absolutely necessary, but it is recommended for a healthier tank. If it isn't cured there can be hitch hikers and other somewhat undesirable things that come with your rock. Also, I think it can pollute your tank and raise ammonia, nitrate, or nitrite. I don't really remember which one. Sorry if that doesn't help much. Others will post though, I'm sure.
Since you are just now setting the tank up, you shouldn't have to worry about any of the problems of introducing uncured liverock to a tank. It will actually help cycle your tank.
You might scrub the rock down with a clean (preferably never used) brush (like you would use to scrub dishes) so you knock off any debris (dying sponges, algae, etc.) so as to not add a bunch of dead junk to your water.
You should have plenty of time while you're waiting on the cycle to complete to find any possible unwanted hitchhikers.
You do want to wait about 3 weeks after your live rock is in the tank before you add anything. If it is fully uncured it will produce high levels of ammonia that will kill stuff in your tank. After about 3 weeks, all of the dead things on the rock should be gone, or at least enough of them will be gone so that you don't have to worry about things dieing. If you buy your rock from a pet store, it will usually be at least partially cured and then you don't have to scrub it or wait as long.
I am not quite sure what you are trying to clarify. If you are clear on the ammonia/nitrite/nitrate cycle for curing a tank and how live rock plays the catalyst for this then I assume you are asking if you can add uncured rock directly to your new tank as Animal Mother addresses in his post above. If you are not yet fully familiar with cycling a saltwater aquarium, there are numerous posts that a search here or on any reef forum or any sw book will provide a detailed explanation.
Personally, I always put new rock (I only get fresh uncured) in a large Rubber Maid type bin with circulating saltwater that I keep set up in the garage (there are some luxuries to being old(er) and getting back into this hobby later in life ). After leaving it for a day or seven, I scrub it less gently than Animal Mother suggests. The reason for the soaking and the delay is to allow for easier removal of anything that will die because of shipping. Sponges, in particular, need to be removed even for a new tank. If I am starting a new tank, I put the soaked and freshly scrubbed rock into the tank IF the rock SMELLS clean (oceany smell, not lack of smell). If there is a strong stench, I repeat the scrub and soak process after finding the dead matter (if it smells strongly, something is dead, usually tanish colored sponge). This process allows for a slow but steady cycle. It also means that you must slowly add your critters (not an octopus) since your tank will appear to have cycled quickly but you will have to continue building on the bacteria growth to be able to handle introduced bio load.
Others like to put fully uncured rock into a new tank to take advantage of the high ammonia given off by the dead and dying contents. The initial cycle is longer and nothing can be put into the tank until the nitrites are gone (often a real struggle for the last of them) but at the end of the initial cycle time, there is a strong bacteria presence that can absorb an immediate bio-load.
Both methods take about three months to build up enough bacteria to handle larger critters. I really don't feel that a tank is fully cycled (read that as stable) until it is about a year old. I prefer the first cycle method because I can add interesting clean-up crew during the cycling process where the second method keeps the tank pretty much leathal until the initial cycle finishes. Unfortunately the first method as well as that nice clean cured rock often found in pet stores can easily result in "new tank syndrome" because aquarists see the nitrites disappear rapidly and think they have a full cycle and overwhelm the bacteria by adding too much too soon, restarting the ammonia cycle. With the first method as with cured live rock, I have seen zero nitrates in as little as two weeks -THE TANK IS NOT CYCLED. If the rock was precured, you may even need to add an ammonia builder (dead shrimp are often used) because there was not enough bacteria in the not quite so live LR (if you never see an ammonia spike the tank is definitely in this syndrome). If the rock has no smell at all, avoid it for the purpose of cycling your tank. If it is attractive and "just right" for the tank, buy it but add additional rock to function as the cycling catalyst. It takes "dead" rock to become "live" rock a minimum of two years cure time in the ocean. The piece I have in my cold tank took over three years to show any signs of changing.