A lot of the bulbous-based species grow horizontally or downward in nature. Some have tight bases, but others have large open spaces at the bases of the leaves that could function as water reservoirs. In nature their horizontal growth keeps water from accumulating in these spaces; ants often live there, presumably providing protection and fertilizer.
In cultivation, if water stays in these spaces too long, the plants may rot. Some plants like this are T. caputmedusae, ehlersiana, mitlaensis, seleriana.
A good resource for Tillandsia and other bromeliad information, including photos of a huge number of species, is the
Bromeliad Encyclopedia of the Florida Council of Bromeliad Societies.