Billboards are simply large commercial posters meant to be seen by many people from very far away. They are always in places designed to be seen by thousands of people a week—sometimes a day.
Billboards on large buildings are (if accessible and practical) a great place to create a large rescue message or telling any survivors nearby where to go if you're scouting for people although it could put your shelter at risk of bandits and looters. Many billboards are on specially designed towers along highways, major roads, and on city limits. These towers are accessed by ladders that don't come all the way to the ground. With a crowbar hooked over the bottom rung (or other improvisation) it may be possible to gain access to the catwalk above. The catwalk might temporarily shelter about ten people from zombie attack. The possibility of falling might be considered an advantage in light of suicide as an anti-reanimation measure. The heights could also offer a low-risk reconnaissance advantage, allowing one to survey an area before travelling there. It can also serve as an adequate sniping position.
A billboard lacks nearly everything required for a permanent base. Billboards have no food or water sources and are exposed to the elements. If you get surrounded by bandits/zombies you will have no way to escape, except by deathly methods. Therefore a billboard should be low on the list of potential base sites.
Billboards are designed to be looked at so if living or camping up there, you may be prone to raider attacks.