Why do cards have different rarity levels in Magic: The Gathering?
The main reasons for rarity are:


To allow for complex cards without overwhelming new players. Rich interactions are part of what make Magic great, but the sheer volume of rules can be overwhelming. By limiting complex cards to uncommon, rare and mythic, it reduces the amount that new players need to worry about it.
To allow for interesting limited environments as well as powerful cards for constructed. By putting limited bombs at rare or even mythic rare, it reduces the effect they have on limited format games.
To sell cards.