Understanding the class tiers is incredibly important to gaining the maximum enjoyment out of D&D. While some people only care about telling a story or acting out a role, it's fair to say most players want their characters to at least be effective, and the only way to tell how effective your character is is to compare them to your party members.
You see, a well played fighter will never be effective in a party with a well played cleric or wizard. It just won't happen. If the fighter is more useful in a brawl than the cleric, the fight was either a)tailor made to screw the cleric, or b)the cleric felt bad to the fighter and didn't fight well. And if the fighter had a chance to do well at all the wizard really isn't doing their job.
Thus, if you want your character to be effective, you shouldn't play a fighter when a wizard or cleric is in the party.
In general, I prefer if every character is within 1 or 2 tiers of eachother. If any tier 1 characters are in the party, the lowest tier in the party should be a 3, and knowing this, if you choose to play a lower tier, you don't have a right to complain when you feel useless or when the rest of the party treats you like a liability.