You'd normally expect a reboot to mean that a studio or film-maker wants nothing more to do with the thing they're trying to replace. It's effectively the equivalent of replanting seeds in a scorched field - a symbolic gesture that lets the audience know that this new thing is exactly that - new and different and not in any way weighed down by the issues of its predecessor.
In strange, rare cases, though, there's one element in the scrapped former generation that is just too perfect for the film-makers to move on from. Sometimes that manifests as ideas or occasionally plot threads that make the jump over the reset, but in the rarest of cases, it's decided that an actor has played the character so perfectly that recasting would be a disservice.
In those cases, you end up watching a wholly new film in a new universe or timeline that has a recognisable character wandering around as if that's just totally fine...
WARNING: This article contains spoilers for Spider-Man: Far From Home. SPOILERS are on the final page.
9. Deadpool - Ryan Reynolds
Movies: X-Men Origins: Wolverine & Deadpool
You really have to hand it to Ryan Reynolds, he's pretty much the best evidence of the old adage that "if at first you don't succeed, try again." For a while, it looked like he was box office poison for comic book movies (what with Blade Trinity, RIPD, X-Men Origins AND Green Lantern to his name), but then it all came up roses with Deadpool.
And to think, his awful, character-betraying role in X-Men Origins: Wolverine would normally have been the end of his attachment to the character. Instead, Reynolds retained the enthusiasm for Deadpool that had driven him to accept the role in the first place (despite the script) and helped get his own project greenlit to restore Deadpool's good name.
And thank God they let him.