It's inevitable in Dungeons & Dragons to hit a point in leveling up where a player is unsure whether they should take an Ability Score increase or a Feat. Many Feats are geared toward the more martial classes, so spellcasters may be tempted to avoid them altogether. However, there are a few Feats that are too good to pass up.
Depending on what a player wants to accomplish with their spellcaster, Feats may be a necessity, and they can have a huge effect on how Wizards or Sorcerers play the game. D&D's spellcasting classes can become the most powerful characters, especially at higher levels where they can end massive combat encounters with a single spell. While martial classes often require Feats like Polearm Master or Sharpshooter as buffs, there are some that truly take casters to the next level.
War Caster is an option ideal for anyone who mixes the magical with the martial, but so good that any spellcaster could benefit from it in combat. It allows for advantage on Constitution saving throws to maintain concentration, the ability to produce somatic components even with a weapon or shield, and the option to use spells for Opportunity Attacks -- situations that will frequently arise in the thick of combat. However, even simply making it easier to maintain concentration makes this essential, as that can make or break encounters.
Outside of combat, Ritual Caster is invaluable to any magic user who does not already have this. Considering how D&D's the magic system works, conserving spell slots is crucial. This means the ability to minimize the number of spell slots burnt by performing rituals greatly increases a caster's utility. For the same reason, Feats that offer a free use of spells a caster might frequently use, like Fey Touched does with Misty Step, can free up space and make all the difference.
Perhaps the best hybrid Feat that's helpful in and out of combat is Telekinetic from Tasha's Cauldron of Everything. This not only allows casters to free up one of their hands by telekinetically holding objects (which is invaluable when somatic and material components take up hands), but the option to Shove as a bonus action means that casters can establish distance when in melee range. If a telekinetic caster can shove an opponent five feet away, they can utilize their movement to flee without provoking an opportunity attack, meaning they can reestablish range when they need.
Almost any spellcaster needs to take War Caster or Telekinetic. The former helps maintain spell-based combat while in melee, while the latter reestablishes range for casters who flounder under martial conditions. While ability score boosts can certainly help spellcasters get stronger, at a certain point, Feats are a much better way to continue to optimize a character so they can dominate the game.