Naw, you don't really NEED a cycling jersey. I would say you need cycling-specific shorts, shoes, and maybe even gloves before you need a jersey (you can make up your own mind about helmets, if you live in a jurisdiction that doesn't require them).

But a cycling jersey does have certain advantages/useful features:
- back pockets that you can access while you're moving, and that will hold stuff without the jersey sagging,
- cut close enough that it doesn't catch a lot of air or flap around too much in the wind,
- the best jerseys have a full zipper so you can unzip completely on hot days/climbs, or you are able to remove the jersey completely without having to empty the back pockets,
- recognition value. Shows you are a member of a club, or have achieved a particular status (e.g., California Triple Crown, or National Champion, or done a particular big ride)

People who have been cycling for years will have a closet full of jerseys. Although they don't contribute to comfort as much as shorts or shoes, jerseys are what you usually keep long after the shorts and shoes have worn out and been chucked.

If you don't feel the need for a cycling jersey, don't even bother. Life is too short to be cycling in ugly or mundane jerseys you don't even care about. But at some point, you will find yourself attracted to a specific jersey for some reason, whether it's because you've joined a club, or done a ride, or whatever. That's when you buy the jersey.

Luis