By having your wifi unsecured, anyone can use it. This has benefits and problems.
Benefits: people who come over will not have to enter a password to use your internet on their devices, you do not need to remember a password as well. (This point can be negated because often your computer will remember the password of a secured network and auto-log you in.)
Problems: people can use your internet for scrupulous purposes (illegal downloading, trafficking child porn) and the trail for these activities leads to you. Also if you pay for bandwidth or if you have a limit of number of gigabytes you can transfer each month, you may find yourself with a large bill and be forced to pay it.