Why use wireless? simple...

For people who rent, fixing up a wired network simply isn't an option (assuming you want your deposit back). Even for those that don't, setting up the wiring is non-trivial if you don't want your house to look like a workshop with wires stapled to the skirting boards. Digging channels in walls and drilling holes in floors for routing the CAT5 is not exactly the work of an afternoon - and nor is plastering over the aforesaid holes afterwards!