For many people, romantic chemistry is a really, really important part of any serious and potentially long-term relationship. It can also be important (sometimes perhaps even more so) when you're looking for something a bit less serious. Determining who you might have a lot of romantic chemistry with and who you might not be quite as romantically compatible with isn't always as obvious as you may like, but knowing their birthday might give you more of a hint. You might want to know the zodiac signs you'd have the most romantic chemistry with, based on your sign, to know who you might have more chemistry with than you may have thought.
If you generally think that your personality fits with the traits that are often associated with your sun sign (or the sign that you likely know as your zodiac sign), then knowing the signs you're supposed to be romantically compatible with could be useful info to have. (Also useful info? Knowing which signs you're supposed to have toxic chemistry with.)
It's worth noting that your overall chart goes far beyond your sun sign alone, which means that you could very well have chemistry with signs you haven't been paired with below. Still, romantic chemistry is a super important element in a relationship, so if you can figure out who you might share that with based on your signs... well, it might not be the worst idea ever.