At D1 and D2 you can sign an ineligible but you're not sure if he'll show up until after recruiting finishes. So he fills one of your open scholarships. At D3 they will show up and can play right away.
If he chooses to stay at your school he takes up one of your 12 roster spots and will not improve until the offseason.
If he goes Juco you get a walkon and the guy goes into the recruiting pool of next season's recruits. They attend a Juco that will be different in mileage than his high school and are recruited from the Juco's location the following season.