Sheffield is one of England's largest city, so ultimately that would explain why a stadium of Hillsborough's size would be built.

There was a time when Hillsborough would entertain much larger crowds on average. A quick review of record home attendances sees a game at Hillsborough featured in the 10th spot.

As of late there have been few games where Hillsborough has been filled even close to capacity, but notably on the final game of the promotion year from League 1 (2011/12 season) Hillsborough hosted the largest crowd in the Football League - including the Championship. This would indicate the club has more than enough fans to justify a stadium of Hillsborough's size.

The highest attendance for a single League match last season was 38,082 (Sheffield Wednesday v. Wycombe Wanderers on the 16th May 2012 in npower League 1) Source.

Despite Sheffield Wednesday's recent poor performances, when compared to the competition attendances have been healthy (averaging 20k+). There is hope that if Sheffield Wednesday reach the promised land more people will show up. With that said, the last time Sheffield Wednesday were in the Premier League, attendances were probably around the 27k mark.

In a nutshell, you can review factors beyond the club's performances. While recent history suggests there aren't enough people interested enough to fill Hillsborough on a consistent basis, the stadium is older than recent history.

Beyond Sheffield Wednesday, Hillsborough Stadium was often used as a neutral venue for cup matches, and was a venue for the 1966 World Cup, Euro 96, and England's failed 2018 World Cup bid.