Why UrBex ?
Altho most people might say 'Because its there' the real reasons of why people go on and perform Urban Exploration trips (or UrBex in short) are different from person to person. Some could have an interest in archeology, some have historical motivations, while even others claim that they are driven by the architecture of the place or even just the pure thrill of it all.
On a personal level it's about history mostly... Visiting old factories with old machinery that has not been breathing, hissing, puffing and spinning for years - sometimes party disassembled by movers or even vandals. It allows me to show how people did things in the past - we did not always have modern maglev trains and microwaves you know.
The Wikipedia Entry on Urban Exploration goes into more detail about the reasons and variations - So you might want to read that article as well