When does a movie need a sequel?
Well. Its based on several factors.
Is there enough material in the first movie that can be used in sequel?
Is there a possible story arc or can be a series of stand-alones?
Is a sequel(s) really necessary?
I raise this question because I've been watching Jaws on Blu-Ray which I do consider to be a superb movie in it own right. But due to it success and also being one of the movies to create the Summer Blockbuster someone felt a series was necessary. Jaws should have worked as a stand-alone movie where no sequel necessary. Yes, Jaws 2 you can see what happen after the events of the 1st movie and that would be fine. But 3 and the Revenge were just totally unnecessary especially since the Revenge... so wrongly titled considering that the sharks from 1 and 2 were dead and that the fourth shark was having this "psionic" connection with Ellen Brody and then you have the shark roaring and then exploding by being stabbed with the bow of the ship.
Indiana Jones is great example of series of movies that can be stand-alone movies because they don't follow a sort of arc. For example you can watch the Temple of Doom, Kingdom of the Crystal Skull, Last Crusade and Raiders of Lost Ark in no particular order because they are not tied to the 1,2,3 order.
Star Trek is different case entirely because there is just a wealth of material to use in grandier Star Trek Universe. For great example is the "Genesis" arc where its Space Seed, the Wrath of Khan, the Search for Spock, and the Voyage Home. Do you really watch Space Seed to follow the plot of Voyage Home or the Wrath of Khan. No, it not necessary but it does give background to the overall arc. Where as Generations can be considered part of the "Duras arc" from Next Generation and Deep Space Nine because it would give the viewer necessary background on Lursa and B'Tor. While First Contact is a sequel to Best of Both Worlds and can be considered a pseudo-crossover with Deep Space Nine. But there were two that could have stand more time development 5 and Insurrection because they were the worst of the lot. The Motion Picture was a product of its time and circumstances because every movie studio was looking at 2001.
Now where I do think sequels were unnecessary were the Police Academy movies. Here's why. Police Academy started off as a black comedy during a time when such movies were very popular: Caddyshack, Animal House, Stripes and some others. The 1st one was good, and I liked the 2nd movie and 3rd was watchable. But with each sequel you will notice a lowering of the rating from R to PG-13 then to PG in remaining for 4 movies but also the movies were also becoming tired and formulaic almost outright dumb, I think by the time of the 5th movie some one at Warner Bros figure this could be marketed towards kids and at that same time an animated series and toy line were in the works.
The Marvel Cinematic Universe actually is actually doing sequels the right way where you have Iron Man, Captain America, the Hulk, and the Thor plus the other movies they have in works while separate in their own right but are still tied to together in an overlapping arc which will involve Thanos most likely but still there are little threads and tie-in to the other movies much like in the Marvel comics where in an issue of Captain America there be would reference an incident that had happen in an issue of the Fantastic Four showing its own universe than being self contained unlike a lot of other comic book adaptations.
The 1st Batman series suffered the same problem as the Police Academy movies by the 3rd movie because "Returns" was considered so dark and depressing that Burton was told to take a hike and brought in Shumacher to take over the problem is that they took series in the wrong direction where it was harking back to not only the 1960s Batman series but also the Batman serials of the 1940s where they were lighter and more kid friendly than say the Batman comics of 1930s and 1970s. Batman the Animated Series and Batman: Mask of Phantasm got it right where there is balance where kid can enjoy on one level while the adult can enjoy on another level. Unlike the Bat Credit Card.... A BAT CREDIT CARD.... YOU BASTARDS! Sorry for channeling the Nostalgia Critic, if you haven't seen his review for Batman and Robin, do so it is funny.