Is there a reason to spread your research into multiple journals instead of just one? Suppose one works in field X, and has say two or three manuscripts ready for journal submission. Journal A is a good fit for all the manuscripts, but journals B and C would be appropriate as well.

All else being equal, is there any benefit in having papers in different journals? Does it hurt to have all of your papers, or say a substantial amount of them, in the same journal A? For example, would it be a good idea to submit one to journal A, and the other one to journal B?

I could think of someone saying "I read journals B and C, but not A", so in this sense spreading your research in different journals would make sense. But maybe this is not so typical to begin with.
There is obviously no right or wrong here, one should try to publish in the best suited journal. That said, in fields where there are several options, sticking to a single journal may look a little strange. If you publish in different (but suitable) journals, it may be looked upon as that your research is accepted by a wider set of peers. Some may perhaps also think you have a special connection to the journal etc. 

So the need to publish in different journals should primarily be the focus of the journal. Some people I know enjoy the fact that they are published in widely different journals and some people may see that as your research being more widely accepted (right or wrong).