I assume you mean this effect
The original Flicker, the basic effect is to remove something from the battlefield and return it later on.
Why would you want to do that? Well there’s many reasons you would want to, mostly for experienced MTG players:
1) If done at instant speed, it removes a creature from combat immediately. That means there could be situations where you block with your creature then remove it from combat before it takes any lethal damage, meaning the other creature gets blocked but doesn’t actually deal any damage to it.
2) If used on the opponent, you could temporarily remove problematic creatures, enchantments, artifacts, etc for one turn which may turn the tide of the match.
3) If your creature had counters on it which may be detrimental (for example, fading counters or -1/-1 counters) you can remove the counters or reset the counters by flickering it.
4) if you have any permanents with ‘enters the battlefield’ effects that trigger, they can trigger again.
5) if you have any permanents with ‘leaves the battlefield’ effects that trigger, they can trigger again
6) you can protect your permanents from spells that target it by flickering it
All in all it’s one of the most useful effects to have in the game.
^ if you see this on a creature it can get extremely annoying to deal with
^ a similar effect, except you have to repay the mana cost to replay the creature whereas a flickered creature comes back for free.