It may happen that the combination of software components you find yourself using gives rise to conflicting requirements for the optimization of tuning parameters. If you can separate the components onto different machines you may find that this approach (a kind of clustering) solves the problem, at much less cost than buying faster hardware, because you can tune the machines individually to suit the tasks they should perform.

For example, if you need to run a relational database engine and a mod_perl server, it can be wise to put the two on different machines, since an RDBMS needs a very fast disk while mod_perl processes need lots of memory. Placing the two on different machines makes it easy to optimize each machine separately and satisfy each software component's requirements in the best way.