Bowline's will work
, but greatly reduce the strength of a line. Depending on the type of line and it's construction the reduction can be as much as 50%. Eye splices on the other hand can retain 100% of the breaking strength of the line, depending again on the type of line, and the quality of the splice. Bad splices will still retain about 90% though, so much higher than a good bowline.
In an ideal world you would have dock
lines that are measured exacally to the boat
with eye splices in both ends. This allows you to just leave them in place, grab them as you come in, and slip the line over the cleat. However since most of us don't want to buy separate lines for storm conditions, and regular tying up, or for traveling, it is very common to have an eye spliced on one end, and bring the other end to a cleat. This gives you the strength of the splice, and some variability in leingth when tying up at strange docks.