Spring is a framework whose primary purpose is to allow you to construct and wire together JavaBeans. There's a core component, designed to be lightweight, and then there's add-on components for specialized functions such as ORM support (including Hibernate and JPA).
There's no absolute requirement that an app use Spring, but there are some advantages. For one, since the bean instances and relationships managed by Spring can be controlled by an external configuration file, you can make plug-replaceable components and swap
test components in and out. I've got a dummy emailer in one app right now. For testing it just dumps the generated emails to the log. In production, I'll wire in the production mailer and it can begin actually transmitting mail.
One of the best reasons for using Spring with an ORM - or even raw
JDBC is that its persistence component takes care of a lot of grunt work. There are certain things that you have to do over and over again when dealing with persistent store, and I've seen a lot of sloppy code on that account - especially leaky JDBC connections. Spring obviously can't keep people from writing sloppy code, but its persistence wrappers can at least ensure that the sloppy code doesn't leak connections. Additionally, Spring provides abstract transaction management which meets or exceeds things that would either be even more grunt work or require EJBs, and it does so for environments where pre-V3 EJBs weren't supported.
