Gems are used for summoning but they are also used for other things like refilling your energy, expanding your unit box size (5 per gem), expanding your item slots (5 per gem) and expanding your friend’s list (5 additional slots until you hit 100). You go to the “Shop” tab and go tap any of the increase capacity that you need.
There are also other applications like arena orb refills and etc. So, if you are not spending actual money, you do need to manage all of the free gems you will be getting. Always practice never spending every single gem on summoning. Always save some for expansions, because you can’t work with 50 slots of everything forever.
My tip for gem is save/use as needed based on what kind of player you are.
If you are more F2P (Free to Play, spending almost no money), then save your gems until the next major event for summoning like a Collaboration or Brave Frontier Anniversary. Otherwise, you should focus on your expansions and not go overboard.
If you want a model of a general spending plan, use login gems to expand and gems earned from content for summoning. This is just an example, not a method that should be followed.
If you are more P2P (Pay to Play, not actually paying to play the game but willing to spend money), then spend however you wish in a healthy manner.