I think it does make increasing sense to have one.
I notice with a lot of financial sites, it is easier to login with their mobile app than on a desktop computer, presumably because the fact you are logging in from a known device counts for something.
Lots of internet security relies on you being able to receive text messages. Banking is becoming considerably less convenient for people who don't have a SIM card.
It needn't mean you have to answer at any time. I almost never answer the phone other than when I'm sitting at my desk at home.
I do have a landline number, which has been migrated to VOIP and so actually is answered via an app on my mobile. (The landline can potentially wring on all household mobiles simultaneously until someone answers.) But I've finally made the switch to giving out my mobile rather than landline number to companies, because if there's any danger of them actually calling, there's no point in having DW phone also ring when they do. It only makes sense for the landline number to be used by callers who genuinely don't care which member of the household answers, for example delivery people.
Quite simple, the admin of modern life is increasingly dependent on you have a personal phone number.
I've actually saved money by terminating my copper landline. I get incoming calls on my old landline number for free via a VOIP provider. I spend £2.50 a month on mobile running costs. Even if mobile depreciation took my total cost up to £10 a month, that's still less than most people pay to have a landline, and I get both mobile and VOIP landline for that price. (Admittedly I could only terminate copper landline because my internet doesn't depend on it, I have fibre-optic to my home, which the majority can't get yet.)