Minor correction: Roman emperors weren't commonly deified, during the first century only 4 of them were in fact deified; Augustus, Claudius, Vespasianus and Titus.
So anyway, we are not entirely sure how they were worshipped after they died. We know that they obtained the same kind of priests as the 'real' gods and that the sacrifice practice and celebrations were about the same as for actual gods - for a while. However, even if an emperor was deified, it didn't mean that he was "up-to-date" for an eternity afterwards. Instead the archaeological material shows a clear decline in the worship of a deified emperor as soon as there is another one. Ie., Augustus was mostly replaced by Claudius, who was then later replaced by the combination of Vespasianus and Titus.
Because of the nature of the imperial cult, this isn't quite as surprising as it sounds. While it for us seems like they were neglected soon after they were deified, there was in fact a shifting in generations between the times (Augustus died in 14 AD, Claudius in 54 AD, Vespasianus in 79 AD and Titus in 81 AD).
So what use did being deified actually have? Well, for starters you were sure to be remembered for a substantial amount of time after you died, but you could also have temples erected over you^1 which in itself allows for a more thorough worship practice than just simple altars. It's also a very useful thing for your surviving family. The imperial cult wasn't just a religion, it was politics and propaganda, and having a deified person in the family gave a higher status to surviving family members. I wrote a post about sacrifices a while back which may be interesting to have a look at as well.
[1] This differs greatly between the western and the eastern half of the empire. In the west it was strictly forbidden to erect temples over living emperors, while in the east they could erect them as long as they were shared with a real diety. Meaning that in the east, a deification meant that they could erect temples to the emperor alone, without an additional god. Also, before deification you could only be worshipped by the means of altars, statues and similar.