Without the alert feat, hidden attackers can surprise you, giving them a free round where you cannot attack. They also get advantage on their strikes on you because they were hidden from you.
By taking the alert feat, you negate this (for yourself only). Because you are not surprised, you do not have to sit out one round of combat. Also, hidden attackers do not gain advantage on you, per the feat.
So taking this feat can turn an ambush from your opponents getting potentially 2 rounds of attacks* on you before you can start attacking/evading, into an even fight.
EDIT to add: supprise is handled on a per-character basis, not a team wide basis.
PHB pg 189:
If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends.
PHB pg 165:
Always on the lookout for danger, you gain the following benefits:
You gain a +5 bonus to initiative.
You can't be surprised while you are conscious.
Other creatures don't gain advantage on attack rolls against you as a result of being hidden from you.
*assuming they rolled higher initiative than you