Great question. I would say that, for the most part, they prefer to lease existing locations, as opposed to purchasing the land and constructing the building. However, to address your question, banks (like every other retail establishment out there) are always trying to establish and strengthen their "brand". The signage is advertising in and of itself, and the back offices keep track of year over year expenses/profit for each individual branch, so that if any branch is underperforming, they can close it. This point brings us back to the leasing vs. buying. One provides you an out at the end of the lease, whereas purchasing requires a real estate agent, title company, lawyer, etc.
Additionally, most banks - in particular, larger banks - are trying to establish a more significant online presence. The mobile banking trend is hitting the big banks and they are certainly trying to cash in on what they see as the next "big thing". However, without a physical presence (i.e. the brick and mortar buildings), you risk sacrificing convenience, which is what everyone claims they are shooting for in the first place.