In the heyday of film photography – let's say the 1990s – C-41 film development was available everywhere. For the vast majority of people, photography meant taking photos on cheap colour film and dropping the film off at a convenient local photo processor – often getting it developed and printed in less than an hour. Traditional black & white development was the domain of only certain enthusiasts, and way too much hassle for most people.
However, casual photographers were still interested in the look of black & white photography, for the tone/mood/character of the results. Wouldn't it be great if there were black & white film available that could be dropped off at the same convenient local 1-hour lab and developed/printed in the same way as all the rest of the colour film around? Exactly – that's why these chromogenic black & white films were invented and brought to market, and why they still exist now.
I remember Kodak had a popular one called T400CN, and there was another using the Portra branding – I guess this one was particularly marketed at wedding photographers, allowing them to easily incorporate some B&W variety into their usual C-41 workflow. I guess Ilford's XP2 Super is the only one currently in production.
Addendum: To be honest, these films must not have been necessary at all if the goal was to get B&W results through the C-41 process – at least not in the latter days of film's heyday – because I did once drop a roll of colour film off for developing/printing and received B&W prints back. I obviously pointed out to the lab that these prints should have been in colour. They reprinted the photos in colour, and I kept the B&W prints too, which actually were quite nice in their own right. So it must have been possible for the lab to simply "flick a switch" to specify that the prints should be printed in B&W. I guess they might have taken advantage of this facility to avoid any colour cast in prints, which I remember sometimes happened with chromogenic B&W.