I'm wondering why would an anime that featured several episodes be compiled into a movie.
For example, Attack on Titan spanned 26 episodes that were later compiled into 2 movies. Another example is Death Note, also compiled into 2 movies.
I might understand the release of an original movie with a new story, or a special episode that recaps the series. However, a full movie recapping the series ?!
Is it really necessary or profitable?
It definitely has the potential to be profitable. Otherwise, why would they do it?
Movie adaptions for a successful series have two benefits.
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.
In addition to the financial incentives, a studio may compile a series into a movie in order to release a version without filler, or content that may have been added in for political\commercial reasons. Think of it as being a director's cut.
It's very cheap to cut a TV show up to become a movie compared to making a whole new movie. This is usually done in anticipation of some form of continuation of the story.
People who watched the show years ago but did not buy it can refresh their memory, people who never watched the show get a chance to dive into the franchise. Fans get to experience the show on the big screen.
There's really no downside if the franchise is popular enough.
Other answers cover the modern concept nicely, but as someone whose first experiences with anime dates back to the world of media that existed prior to streaming, Blu-rays and DVDs, the reasoning is simple: It was not practical or economical for studios to pump out VHS episode collections back in the 1970s and 1980s.
I first got into anime when trying to get episodes of old English versions of anime I grew up with: Battle of the Planets (aka: Science Ninja Team Gatchaman) and Star Blazers (aka: Space Battleship Yamato) as well as old Space Pirate Captain Harlock episodes I would occasionally see on local UHF stations in NYC back in the day.
Needless to say in the late 1980s and early 1990s, there weren’t many quality or comprehensive releases of vintage anime like that on VHS past a few official compilation “direct-to-video” releases.
So while nowadays, I can easily purchase DVD or Blu-ray set of all of the episodes of any of those series, back then relatively short official compilation VHS releases is the best anyone could do.
Thus, things like the Gatchaman The Movie (1978) were compiled and released in theaters and eventually on home video. Whether compilation “films” like this were good or bad, for most people at the time a “movie” like this was the only way to rewatch the characters of Gatchaman at home on their own VHS players.
And sure, fandom-based tape dubbing circles existed. But for the average fan who just wants a bit of nostalgia, compilation “films” like this worked well.
I don’t believe there was ever a Space Battleship Yamato video compilation film that was ever released in this way, but the first few Space Battleship Yamato movies were basically a retelling of the exact same story told in the multi-episode TV series.
Ditto with Space Pirate Captain Harlock but I cannot recall the exact name of the VHS release but among fan circles that VHS compilation of a handful of TV episodes was considered a compilation made for “kids” since it summarized and simplified a lot of the plot-lines to make it all more self-contained.
