What are the advantages of watercolour paper over sketch paper?
Watercolor paper is, normally, quite a bit heavier and made from cotton. Basic watercolor paper, 140lb, is okay for practice and will tend to buckle unless stretched. The expensive 300lb paper is where professionals tend to land. In any case, the paint pools and absorbs differently than normal paper, and gives you options around blending, washes, etc. that you can't otherwise get.

Traditional paper, made from wood fibers, is much thinner and absorbs the pigment faster, will generally buckle and warp rapidly and is much more likely to rip and tear when wet. It's just not designed for that.

In any event, sheet watercolor paper can be had and you can always cut it into smaller sizes for practice, which is pretty common anyways. The cheaper option will only frustrate and disappoint, it's already a challenging medium, don't make it hard on yourself.