So you can replay then over-and-over again just like the TV shows on DVD
Imaging before Mission replay was enabled. After playing a mission there was no option to revisit the location or try a different tactic, see what you may have missed? Replaying them as a great feature because you can play at your own leisure.
I actually preffered Mission Replay before the Level Adjustem was introduced. I could just explore everything and swat NPC's aside
Replay also allows one to revisit missions for the purpose of completing accolades. Primarily however, it is used to get level appropriate gear without spending EC or dilithium. For example, at VA, if you were stuck or were interested in trying out new weapon types you can get a decent uncommon photon or quantum torpedo launcher from one of the early Klingon Front missions and alkl types of phaser, plasma,tetryon and disruptor weapons systems across the various 'fronts' - probably even more like chroniton etc.
Story missions give some of the best experience in the game. Replaying a couple is usually faster than some random grind when you hit a gap between story content.
For featured episodes, Cryptic likes to throw some rewards on replay only, like the crafting unlock on Second Wave or the Breen set, so there's quite a few missions where the rewards you might want most aren't available the first time through.
As a new VA, you can get good gear from these missions, like Covariant Shields [Cap]x2, and other mark-specific gear like the Hargpeng or whatever torpedo, Overcharged Plasma Sniper Rifle, and so forth. There's also The Long Night, which rewards a purple ground weapon.
"Skirmish", the first mission in the Devidian series, is relatively quick to play and awards (among other items) a leveled purple Personal Shield that gives defense against Psi. Handy in general, and especially when going up against Undine or Remans.
Well, for "Second Wave," when you replay you get more reward options. Instead of just the two guns, you can also choose a recipe to let you craft the Jem Hadar boosting items, a box of the boosting items, or a Mystery Lock Box.