Just to expand on the answers you've got pointing out how important mod energy is, consider the situation where two identical teams with similar gear levels are battling. There are only factor three factors determining the outcome of the match are 1) not miss playing and making mistakes, 2) the very very small chance of having awful luck in the elements of the match that are random chance and most importantly 3) the quality of your mods vs the other teams mods.
In some cases that boils down to how much speed you have on your mods vs your opponents mods but there are plenty of cases where multiple other stats that come from mods are also important. The quality of your mods however is a function of chance since the stats the appear on a mod and subsequently increase when you level and slice your mod are random.
As an example (with very rough numbers): If you upgrade/slice to gold a single mod you get that started with speed as a secondary stat you have a slightly better than 0.36% chance you'll get over 20 speed on it. If you upgrade/slice 250 mods you have a bit over 50% chance to get a mod with better than 20 speed secondary on it (not to mention a whole host of 10+ speed secondary mods too). The more you upgrade the better your chances and your mod inventory gets. But that only considers the one secondary stat of speed. Ideally you want that secondary stat to be on a mod with the most optimal primary stat and set bonus and if possible 3 other suitable/useful secondary stats.
So as others said, upgrading mods is a never ending process as you can always improve on your current mods. That's why if you have the resources most people recommend the 3x 50 crystal refreshes as a minimum every day, divided between acquiring new mods and slicing materials as needed. Of course at lower crystal income that's pretty impossible to sustain but should be a goal if you start climbing high in arenas and increasing crystal income.
This is a useful beginners guide to modding that may be worth you looking at... https://gaming-fans.com/star-wars-goh/swgoh-101/swgoh-101-comprehensive-mod-guide/