Xerox mostly operates in Enterprise printing now, and believe you me, campanies still print a lot. Lexmark operates both in consumer and entreprise printing, so a bunch of new customers for xerox.
The newly joined company will probably be able to do a best of breed product roadmap in HW and SW going forward.
Also, I do not know, and I am to lazy to ask, but there may be geography complimentarities (i.e. markets were Xerox is tronger but lexmark was weaker and Vice versa), that would allow a best of breed approach in each.*
Finally, economies of scale with suppliers kick in, and personnel may be made redundant, saving costs.
* An example I remeber vividly was the DEC-Compaq and Latter compaq-HP merger. For some reason, the DEC guys in colombia survived both, while the dec and compaq guys in Venezuela were Decimated. Seems like DEC was the stronger of the three in the colombian market, both in Servers and in User PCs. Those guys were hungry for sales.