I was hesitating to reply to this but what the heck.
First, I'm going to tell you to use your resources. Google, diablofans, reddit, etc. The information you seek, in as much or as little detail you want, is already out there and easy to locate. If you want more detail than what I'm going to give, find it yourself.
To answer your question, yes, without a doubt, there is a point to leveling any gem you are going to use past 25. 25 is the point at which a gems secondary effect activates, but all gems improve beyond this point. Some are marginal, or questionably useful, like Bane of the Powerful (improvement adds time to the duration of the buff, but you wouldn't use this outside of farm content, and by the time you can level it to, say, 50 or 60, you are killing stuff so fast the buff would always be up with the rank 25 version - just as an example). Bane of the Trapped, however, simply gives an increased damage bonus every level, leveling it is always going to make you incrementally more powerful.
Most gems, assuming you are using them, get more effective with each additional level. The upgrades are marginal from one rank to the next, but if you compare a gem that is 20 levels higher, you'll notice a stark difference.
As an above poster mentions, there are several gems which have hard caps at various points, above which you cannot go. This makes sense looking at the individual gem, since you obviously can't get more than a 100% chance to proc something, etc.
Finally, there is a new mechanic in the recent patch, intended for players on the higher end of the gearing spectrum, where you can use a leveled legendary gem and other mats to permanently add a stat to an ancient piece of gear, and the higher the rank of the gem, the more stat you add.