The primary benefit is in the guild task reward system. Every player is able to donate gold toward the completion of a set of tasks that reset every week. As each task is completed, EVERY player in the guild, regardless of whether they donated toward that task or not, gets a reward of glory, maps, souls, gems or keys (gold, glory, gem or event). The return on investment is really good, especially at the low levels of guild tasks. So, if you can find a guild that suits your activity level, then everyone in the guild benefits from everyone else’s contributions. The more people there are in the guild, the more cost-effective the donations are.
A good resource to see what the rewards are is here:
One example to illustrate: If you joined a guild that had 30 people in it, and someone donated 5000 gold to complete the level 1 Brown task, every player in the guild would get 30 gold keys. That’s 900 gold keys for 5000 gold, or about 5.5 gold per gold key. These normally cost 300 gold, so that is obviously a bargain. Even if there were only 5 people in the guild, that 5000 gold would still buy 150 gold keys, for a net price of 33.3 gold per key. Still only about 10% of the cost of buying gold keys with your own gold.
This works best if everyone is contributing, so there can be “free rider” problems, but that is up to the guild leader to deal with. Also, the ROI decreases as you move up through the levels, and they are capped each week, but this is still the best source of mixed keys in the game and you’re missing out by not joining an active guild. In a lot of guilds, the only participation required is to contribute something toward these tasks.