There are a variety of reasons they've given to justify their plans. These include the need to make people develop a notion of national service, kind of like the "compulsary volunteerism" oxymoron idea that was being floated by some (including Colin Powell) a few years ago.

I've heard others, primarily some Democrats back in the 1980s, who wanted to reinstitute a draft because they though it cost too much to make military service attractive. They longed for those bygone days in the 1960s and early 1970s when a Private (E-1) made $90 a month.

I've always opposed the draft for many reasons, but none more than the prohibition on involuntary servitude (13th Amendment) should be necessary.