As I always said on railforums, this project was always about extending commuter belt from Birmingham to London from a political point of view. They just needed the north to agree to partially fund it.
Having regularly worked for companies within an hour's travel from London, it makes working on projects within London viable (be that with with other companies or actually on construction projects within London).
With the lower office costs within regional towns and cities the cost of paying a few key staff to travel to London over paying the office space for a lot of staff soon becomes worth the extra travel costs.
As such it gives regional business access to projects which otherwise are only open to those within London and the South East.
The reality is that HS2 could allow business to operate from places like Manchester like they do from places like Basingstoke, without needing to have large numbers of people travel every day to work within London.
…and the south. Waving from Southampton.
Southampton to Newcastle journey times (before the Eastern leg for cut) were due to reduce by about an hour - in doing so quite probably removing the need for that domestic flight (as it would put the rail journey time at about 4:10 hours, which is 1:40 Southampton to Paddington, 2:20 Euston to Newcastle and a 10 minutes extra allowance for changing trains at OOC) as it currently operates twice a day each way, so there's a fair chance that much of a modal shift would render that route (at least) less attractive.
Whilst there will still be some who wish to fit the early start/missing the whole day would put people off if they could travel at a more convenient time for them (say leave at about 10 and arrive at about 2). Also it's not uncommon for people from further north (like Winchester, Basingstoke, Farnborough and the like) to use Southampton to fly from, by being that much closer to OOC their journey time comparison is going to significantly favour going by train (30 minutes to Southampton would mean that whilst there journey time from there to Newcastle is only 30 minutes faster, the air route would also be 30 minutes longer making the gap smaller).