What are birthdays for?

[DrakenKin]

I thought any gift would boost their liking, but that's not it.

What's the effect of birthdays?

[Suzaku]

Giving a gift on the person's birthday will give 8X the normal relationship points.

[dino_inc]

It multiplies the friendship point values by 8, so it multiplies negative values too.
http://stardewvalleywiki.com/Friendship

[General Profit]

Giving someone a bad gift/gift they do not like on their birthdays will not make them like you more :P

[Everfreefire]

Birthdays are for not pissing off your spouse when you realize it takes forever to raise friendships without gifts.

[I Kinda Fail]

Most people have universal likes, like beer, wine, fruit, flowers, etc. There's exceptions to every rule, but...