At a high-intensity single-runway operation like LGW (in normal times the world's busiest single-runway commercial airport), less profitable services occupying valuable runway slots gradually get squeezed out in favour of more profitable replacements. That is what happened to the LGW-MAN route which was accounting for a typical eight slots per day, some of these at lucrative peak times. MAN wasn't specifically a 'bad' route - but increasing demand for slots meant that higher profile choices were eventually given priority. But now COVID-19 has changed everything. Suddenly, LGW has been shorn of a significant proportion of its former scheduled programme. Flights are either duplicated by services from LHR, or otherwise not viable to run at their former frequencies. Some carriers are in financial difficulty and have scaled back. Others have moved services over to LHR. Major operators are unlikely to return straight away at their previous scale of operation when business starts to come back. But valuable slots risk being forfeited under 'use it or lose it' rules if they go unused for too long. So it makes sense for a carrier such as BA to 'slot-sit' some of their slot portfolio with short routes which don't cost too much to run and which may bring in some revenue to help offset operating costs in the meantime. LGW-MAN is perfect for this purpose. It may not be a long-term proposition, but whilst it is offered some will find it useful (including me). I'll certainly make use of it if it does become available again, though I appreciate that the slots will likely be redeployed to a more prestigious service when demand returns sufficiently.
Out of interest, Manchester lost its scheduled services to London City for similar reasons. As the popularity of that airport grew, it's scarce peak runway slots became increasingly valuable, and eventually the MAN service gave way to more profitable replacements.
One more thing. The train is not a great option between MAN and LGW. There used to be a through service from Manchester terminating at Brighton, but I don't think that has run for a while now. So that means train from Manchester to Euston, taxi or tube to one of the other London stations, and a second train down to LGW. It is a slow, expensive and cumbersome journey - especially with luggage. Not a competitive option at all. And flights to LGW aren't useful only for flight connections: a decent stretch of the South Coast and South London will be much easier to access from Manchester for as long as the service lasts.