What separates so-called “social games” like MMORTS games from the more conventional genres - even if those games have multiplayer elements - is that deep-level player interaction is built into the structure of the game. Simply put, if you are not reaching out to other players, you are not getting full value out of your playing time.
Social games are balanced around the idea that players will form partnerships, clans and rivalries. In short, while it might be possible to play these games all by yourself, you will find it unsatisfying compared to the full experience.
One major benefit of forming an alliance with another player is the obvious one: That player won’t be attacking you any time soon. If you have an intimidating neighbor, it’s better to be their friend than their enemy.
But there’s so much more to it than this! A good ally can loan you resources, or even just give them to you, to help you be a better ally to them in return. They can help you guard your holdings while your attention is elsewhere, and help scout new targets.
Having allies gives you people to talk strategy with, letting you pool your knowledge about the game, and with whom you can share jokes, funny stories, and tales of your glorious victories.
When things go wrong, you’ll have someone with whom you can commiserate with and plot your revenge.
Most strategy games of this type put a limit on the number of allies a player can have, so choose carefully. If an ally isn’t working out, you should tactfully terminate the alliance. After all, ally slots are valuable; but remember that a spurned ally can turn into a deadly and persistent enemy! Ideally, your allies should be people with whom you are already friends, in real life or through the game. Offering alliances to strangers will likely be unsuccessful or counterproductive. Make an effort to get to know a potential ally and ingratiate yourself with them.
Once you’ve established friendly relations, and demonstrated to the other player that you will be a useful partner; they are more likely to accept an alliance from you. Finally, consider whether you and your ally are compatible.
If you’re just starting out, don’t pester powerful players with alliance offers: if you don’t have much to offer them, why would they accept?
Pick your friends and make sure there is mutual benefit from this alliance!