Guilds provide more aids, trades without having to pay an FP, participation in GvG or GE (depends on the guild. Some are active in these but some aren't, or only in one or the other). Also, guilds usually have GB swaps and sometimes if there is a good seller in the guild they will offer a lower price.
This would be better if this was moved to the questions section, if any moderators like @Stephen Longshanks could do that. The Guild hall is where players advertise their guilds, or say they want to join one.
So upsides of a good, active guild include:
Guild expeditions (very essential for a new player in terms of fps and troops provided)
Trade partners (especially since you don't have to pay 1 fp per trade)
Easy access to goods sellers (if they are in your guild)
Convenient group of people who will help answer your questions
More people to aid, and more people who can aid you
Group of folks who you can start arranging swaps with, or you can participate in a GB swap thread
Also an easy way to make some small talk and enjoy some of the more social elements of FoE
Some downsides to being in a guild include:
Gotta follow all the rules of the guild. Of course, what the rules are depends on the guild you are staying in, like participation in GE, GvG, GB sniping restrictions, etc. Of course, you should only join a guild where your playstyle conveniently fits with the rules of the guild
Can't guild hop (although that's a non-issue, and you could just spend a week guild hopping if you are established in the guild)
Possible drama?!? (also a non-issue, as most guilds will sweep that away)
As you can tell, it's a very obvious choice that joining a guild will accelerate your growth and enjoyment in this game. You would be purposely handicapping yourself if you refused to join one.
You get free trades and a decent guild will usually help members acquire and level GBs. There's GE and GvG participation.
Downside is the possibility of a guild culture not to your liking. Some guilds can have a pretty strict set of rules and if that's not how you want to play it's not a good thing. Other guilds kind of wither away if the founders lose interest in the game or a particular server and inactive guilds don't deliver the full benefits.