Even for the most stacked team in the league, it would be difficult to fulfill all of your roster ‘wants’ when you only have six draft picks. And doing so with a quality player that you really feel strongly about is even more difficult.
32 teams just drafted 255 players and probably signed another 300 or so as undrafted free agents, but even with the vast influx of personnel onto 90-man rosters throughout the NFL, teams are still going to feel as though they have significant needs remaining, which they will have to seek to fill utilizing unrestricted free agency.