The main thing that La Paz can boast is that it’s not as bad as other Latin American capitals. For the most part, it’s a safe and cosmopolitan city with a decent economy. The government is situated here and most foreign embassies. It’s considered the center of the country.
There’s not much of interest to do in the city itself, but it is a good place to organize supplies or get a guide into the mountains. Most international flights arrive into La Paz, and it’s the obvious next stop after Copacabana for those coming from Peru.
Fortunately, it’s possible to get a decent meal in La Paz (about the only place in the highlands where that is the case) and hostels will usually have hot showers. There are some bars in the city, but nothing spectacular. Most travellers will spend a day or two in the city checking their email on reasonably fast computers and organizing a journey to the mountains.
La Paz is also a decent place to buy alpaca and wool clothes, though Peru is better.