Why be stronger? Many reasons, but it boils down to health and being strong enough to do activities that you otherwise wouldn't be able to do, both now (such as hiking or cross-country skiing) but also 30-50 years from now (not breaking a hip bone when you fall in the shower, for example).
For me, lifting heavy weights is a great counterpoint to spending time sitting still in front of a screen. Humans weren't made for that, and most humans nowadays are, to put it simply, weak (compared to what we were evolutionary selected for). As much as we would like to, we aren't just brains on a stick, we are also animals with physical demands.