It definitely has the potential to be profitable. Otherwise, why would they do it?
Movie adaptions for a successful series have two benefits.
- Avid fans will watch it, even if it is just a giant recap
- People who don't want to invest 26 episodes' time into the series will watch it
The latter is especially true as more casual viewers will have heard the name before from the shows' popularity and decide to get up to speed with it through the movie.
Of course, every instance is different and there may be series that flunked dramatically or performed exceedingly well.
In the case of live-action adaptions like Death Note, the audience spreads a lot wider and can be marketed to people who wouldn't go near anime usually.
I can't find any sales figures but I'm pretty sure it's a good investment for popular shows, but every venture is unique.
Whether it's necessary is up to your personal opinion.