The exact same reason for why we need funding to improve cycling/public transport.
Improve accessibility, improve safety, make it more attractive, quicker etc. When you consider the number of pavements in poor condition (uneven, potholes, too narrow etc.), unfriendly road junctions with poor accessibility, or so wide that it takes age to cross to the other side, inadequate routes across large roads like dual carriageways or large junctions etc., lack of enforcement against pavement parking etc. - you can soon start to see plenty of things money could be spent on to make walking a more attractive alternative to the car.
I'm someone who will naturally look to walking as my first choice way to travel, and I'm reasonably fit and mobile - I'll put up with crap crossings, crap pavements, unattractive underpasses etc. - but I'm probably an outlier. A lot of other people will quickly be put off with walking on city pavements and jump in the car instead.