People love to add mobility to their fun. This is why about half of all boats sold in the U.S. include a boat trailer. Many hook up the family boat when headed out on vacation; others want the ability to go to another river or lake if the fishing is thought to be better there; some just want to go where there's less (or more) boating action or a shorter launch ramp line. A trailer is also the device where a boat can be stored when not in use, and doing so allows owners to avoid paying for in-water or stack storage fees.
With today's trailers, boats up to mid 20-footers can be easily towed. Of course, larger boats can also be taken along, but they'll need more towing "pull" that may be more than what's rated for the family car or SUV. Remember that a boat being towed will weigh more than its "dry weight." Add in the estimated weight for fuel, gear and the trailer itself. As long as the capacity of the tow vehicle is adequate, a towing hitch can usually be added. For those who plan to tow, most vehicle manufacturers offer "towing packages" that can be added when ordering a new vehicle.
Learning to tow safely is an acquired skill, just as is learning to operate the boat. For those buying the boat and trailer as a package, ask the seller to provide introductory basics on both boat handling and trailer operation, preferably at a launch ramp. Unlike on-land trailers, an important operational dimension of boat trailering is launching and retrieving that includes a number of steps. After a few practice sessions, the process becomes routine, but many launchers follow a checklist to insure success. These checklists can be found in the trailer owner's packet and are sometime affixed to the frame of the trailer itself.
Driving laws include special rules, including registration, for those pulling trailers and are different from state to state. Details are usually available at driver's license issuing stations, from the state department of transportation (or agency charged with driver licensing) and often posted on the state government's web site.
A good resource for trailering information, tactics, selection and tips is published by Trailer Boats magazine.