I think the biggest benefit of a Guild is the friendship you have with the other Guild members. Practical benefits are that you can trade goods between guild members without paying valuable FP for each trade, also a guild tends to have a variety of people at different levels who can help and advise you on all matters. In most guilds you will also receive a lot of help towards any Great Buildings you construct and many have a GB group where you each donate points to each-other to help gain more Blue Prints.
Hope this helped you a little
PS: Feel free to friend request me (keancy) and if you are looking for a Guild and you are female please come join us, we are a female only guild in Dunarsund, Amazons of Light