Factions are only for strat, clans are for other aspects of crpg. In a faction you can easily trade gear/troops/money between each member, in addition to seeing where they are on the map at all times (unless they are inside a fief). Also, if you get attacked and aren't around, then one of your faction mates can accept people to the battle for you, or lead the battle on your behalf should you want them to. For the most part, factions on strat are made up of a crpg clan, and some larger factions are made up of multiple crpg clans with perhaps a few additional members. Having more guys on your side means having more guaranteed mercenaries to fight in your battles as well. Basically, real wars aren't waged by single men, and neither are they in strat, although it is arguably possible in strat, just harder unless you have a lot of friends. But if you had a lot of strat-active friends you'd normally be in a faction with them.