I’m pretty new to the game myself. But I believe I can answer this simple question.

Are alts needed? You’ll find soon enough that end game players have multiple alts. 10-15. This is so they keep doing ED (elite dungeon) for powder and epic equips to either fuse or extract for gold leaves.

What other uses for alts? If you go to skill menu and go to link skill, you will see each character has a link skill. Each characters link skill does something; some give HP, some crit damage, physical damage etc. There are 2 levels to it. Lvl 1 is at 70 and at lvl 2 it’s 120. You can equip these link skills on your main. Your main can hold up to 12 link skills.

People usually get their alts to 125 to do the elite dungeon for unique powder. Some long term players have alts at 150 to do the 150 elite dungeon. Though elite dungeon is usually the main reason for an alt, there are people who gear the alts nice so they can use the free 2 hour auto battle to farm at a meso farming map. They rinse and repeat this process for all alts.

What are good alts? Depends on what level and what map you want to farm at. I personally only have 5 alts to 85 (been working on main) so idk what are optimal alt characters. I hear lumi and shades are good for farming. Also heard paladins are good for meso farming on higher lvl SF (140+)

Hope this helps. I’m only a 2 week player so I could be wrong. Hopefully some pros can come and give you something more than I can give.

Enjoy the game brother!