It's bit like asking why would you use a truck when several vans can do.. and be more "horizontally scalable". Mainframe is a really big server, and it is actu

It's bit like asking why would you use a truck when several vans can do.. and be more "horizontally scalable". Mainframe is a really big server, and it is actually plenty elastic. You can divide it into smaller logical servers (LPARs). The only downside is price. But I think for larger installations and certain workloads it will come out similar with a server fleet.