A maintenance plan gives you free upgrades to new versions of the software and priority support.
Without a current maintenance plan you would have to buy the software again when a new major version was released if you wanted to upgrade to it (minor version upgrades for the version you purchase are always free regardless of maintenance plans).
Please note that a maintenance plan can only be purchased at the same time as the product it is for, it cannot be purchased separately at a later date. To avoid any unwanted or unexpected bills your maintenance plan will not be renewed automatically when it expires - you will be notified when the plan is nearing its expiration date and can choose to renew manually at that time.