One reason, I'd imagine, is just simplicity of code reuse -- less chance of miscoding by mistake. However, there are other benefits. There's an extensive article on the topic. For example:

Excerpt from Perl Slurp Ease by the author of File::Slurp

Fast Slurping

Somewhere along the line, I learned about a way to slurp files faster than by setting $/ to undef. The method is very simple, you do a single read call with the size of the file (which the -s operator provides). This bypasses the I/O loop inside perl that checks for EOF and does all sorts of processing. I then decided to experiment and found that sysread is even faster as you would expect. sysread bypasses all of Perl's stdio and reads the file from the kernel buffers directly into a Perl scalar. This is why the slurp code in File::Slurp uses sysopen/sysread/syswrite. All the rest of the code is just to support the various options and data passing techniques.

That said -- knowing the fundamentals is good too. If I'm just slurping a text file once of a known, manageable size, then using the standard idiom would make sense. But once I'm writing that idiom more than a couple times, I'm going to want to code it as a subroutine -- and that's when I'd rather use File::Slurp rather than roll my own each time.