It is a good central place for visiting The Peak District, The Lake District (Asians appear to be addicted to Beatrix Potter), Liverpool, Jodrell Bank, Southport and Blackpool.
It also has its own industrial heritage as evidenced by its brilliant Museum Of Industry and Science. There is The Lowry Centre, The Imperial War Museum North, Media City at Salford Quays, Trafford Shopping Centre, the indoor ski slope whose name I forget plus, of course, as mentioned by you, the football grounds, Old Trafford in particular and the Rylands Centre.
There is the modern metro tram system, which you probably use when you commute.
Despite all this I am somewhat surprised if it has more foreign visitors than, say, the City of Bath.
Is the FOOTBALL & the MUSIC that brings them here..DO NOT KNOCK IT..Brings in MONEY....Been on city breaks in other places? They are cities as ours is....Museums, shopping ,eating.....I have a friend from Germany who loves coming for the shopping, China Town, Curry Mile,always taking in a play or musical.....& gets a chance to see the great places on the doorstep, like The Lakes & North Wales....A LOT of JOBS depend on tourism & conferences in Greater Manchester..More than you would think. In paper today read tourism is worth 7 Billion pa. to the economy of Greater Manchester....
I have no idea. However they may use Manchester as a base to tour North West England as the Peak District and the Forest of Bowland are close by
You have a great big airport with planes that go back and fore to the US and places. That is a good reason. Also, I have lived in Manchester, and there are places you can get to easily enough which are different from Oldham.