Traditionally kayaks were solo boats, but a tandem or double kayak is a great way for two paddlers to get out on the water together. Working together in one boat two people can typically go faster than each would go alone. This especially useful if one paddler is less experienced or not as strong a paddler. Instead of being left behind by the stronger paddler, they both end up going faster.
Multi person kayaks can either have one large cockpit or a separate cockpit for each paddler. A single large cockpit can be handy if you want to paddle the boat solo as it lets you position yourself in the middle to balance the boat. The separate cockpits allow the use of individual sprayskirts and generally just do a better job of keeping water out of the boat.
