Since the earliest attempts to record sound waves up to the present, the one piece of equipment at the heart of this endeavor has always been the input transducer, otherwise known as the microphone.  Mics are like any other tool;  they serve a purpose and help their owner achieve a goal.  And like many tools, sometimes their owner develops an affinity for them.  Take carpentry tools, for example. To me, a hammer is basically a hammer, yet to carpenters, there are so many wonderful kinds of hammers.  Some have different uses, some are large and heavy, some small and nimble, with different grip material and colors and brands to suit a carpenters hammering "style".  Some even become the beloved "trustly hammer."  Microphones are no different.  Producers and engineers and sometimes even the artists who use them can fall in love with these inanimate yet seductive objects.  They are the primary tool to transfer our passion to tape, and the good ones can even make us sound better than we really are.  We connect with them, like a violinist to their prized stadaverius, or a guitarist and their precious Les Paul, or a drummer to their favorite pickup truck.  But beyond the esoteric reasons for owning a collection of mics, there are practical ones too, which is good, because how else would we justify our purchases to our wives, or stave off buyers remorse? (I for one, have never experienced buyers remorse,  unless the product was remorseful).  Here are some practical reasons why a healthy collection of mics for the most often used types (LDC's, SDC's, dynamics, and even a couple of ribbons) is useful:

1.  On vocals, not every mic is suitable to every voice.  There are some mics that are better suited to male vocals, and vice versa (the Neumann sound VS the AKG sound, for instance).  Voices, like any instrument, have overtones in certain frequencies.  If a mic overaccentuates one or more of those frequencies, a clash can occur.  A singer with a strong forward rise in their natural timbre around 2.5 KHZ probably doesn't need a mic to accentuate that part of the spectrum.  Some of this is science, some is just auditioning different mics until one sounds "right".  I believe in complimenting a voice or instrument's  natural spectral curve as much as possible, and the best way to accomplish that is by trying different mics until you find the best one of the bunch for that particular application.

2.  Because the frequency spectrum of all mics has bumps and dips along their graph (more than appear on the typical frequency plot) that means that one mic is going to boost or cut those frequencies on everything it records.  Recording track after track this way can lead to unneccesary eq'ing, or to a resonant build up or lack of these frequencies.  It is possible to imprint a mic's "sonic signature" on too much of your material.  Some mics are worse than others about this phenomenon (note:  Preamps can do this as well)