I'm not aware of one, but the basic advantages of a farm  are expandibility and
reliability. With a single  server, if 1 machine fails you are down. In a farm,
you can setup  scalability (add more servers as the load increases) and
redundency (if a machine goes down, you have a spare).

How about if I have a single  server at two locations and use one another as
failover. Planning to use DOCAVE along with the planning.

That would work I suppose, but it's a lot more pain to setup  and have things
stay in synch. Doing in a farm  handles all of this automatically...

If it is in a farm, I will only have 1 Front End and 1 Back End, leaving me
in the same scenario.

Is there a way that I can change the modified date on an already posted
file..

The modified field is updated any time you change the metadata or overwrite
the existing file.

If you're talking about changing it to something other than the current
time, then no. It's locked and read-only.

You can certainly create your own field and modify that to your hearts
content.

So, what exactly are you trying to do where you have to change this field?

Can't you use the RPC methods to tweak things like created/modified
fields? I say this since tools like SMigrate manage to keep source
modified dates...check out the SDK for examples and also the SPImport
toolset that's available on gotdotnet -it has source code that you
should be able to re-use.