Does a person need both a landline and a cellphone?

Depnds what they do and what's important to them. My mobile (sorry I'm British, we don't call them cell phones) I carry around for emergencies only, and the odd bit of texting. Any phonecalls I make from my landline because it costs a fraction of the price of making them on a mobile (and it doesn't cut out on you because you've gone through a tunnel or your battery has run out). Theoretically there's nothing a landline can do that a mobile can't, but a landline does it a lot cheaper. You could also argue that we managed without mobiles for many years. In fact we did without telephones at all for many centuries- so if you define "need" as "need to stay alive" you don't really need either.

I suppose that would depend on the person, but functionality-wise, there is nothing a land line can do that a cell phone can't. And cell numbers are just as valid for applications and such as land numbers.

only if they can"t stand being off the phone.
WHAT IS SO IMPORTANT

I use my landline primarily for my dialup internet connection, and for faxing. If I never needed to fax, and I had a cable modem, I'd have no need for a land line.

A land line does, however, have a significant cost advantage if you make or receive a lot of phone calls. Most land-line telephone service charges you a flat rate, which lets you make unlimited local calls, and receive unlimited calls from anywhere, at no extra cost. The service I'm getting even gives me an hour of long-distance service each month for no additional cost.

Most cell phones charge you for airtime, even for local, toll-free, and incoming calls. If you spend a lot of time on the phone, this can get very expensive very quickly. On the other hand, I believe most cell phone service lets you make calls to anywhere in the country (assuming you're in the U.S.) for the same cost as a local call. I remember, several years ago, my boss using his cell phone to make long-distance calls because the per-minute airtime cost on his cell phone was less than the per-minute charge for that same long-distance call on the office's land line.

Bottom line: If you spend a lot of time on the phone, on local calls or on incoming calls, you're better off using a land line.

if you need affordable calls then a landline is the best option. "cell phones" cost a lot more to operate

You don't actually need either. I spent most of the 1980s without any kind of phone.