Same question as title, why?

Strictly speaking, nothing. Stylistically speaking, it's not a good way to write maintainable code because backticks have a (potentially humongous) return value, and you're ignoring it. It's may also not be very efficient, because you have to read in all the lines of output, allocate memory for them, and then throw it away.

Too often people are lulled to writing:

    `cp file file.bak`;

And now they think ``Hey, I'll just always use backticks to run programs.''

Bad idea: backticks are for capturing a program's output, while the system() function is for running programs.

Consider this line:

    `cat /etc/termcap`;

You haven't assigned the output anywhere, so it just wastes memory (for a little while). You forgot to check $? to see whether the program even ran correctly, too. Even if you wrote

    print `cat /etc/termcap`;

this code could and probably should be written as

    system("cat /etc/termcap") == 0
        or die "cat program failed!";

which will get the output quickly (as it is generated, instead of only at the end) and also check the return value.

system() also provides direct control over whether shell wildcard processing may take place, whereas backticks do not.