Primarily for emergency use, but also service quality
A wired telephone service connection or "land line" is a pair of wires connected directly from a switch at the telephone company's central office to the telephone service entrance at your residence, and from there to each telephone instrument in your residence. This is the highest-availability level of service you are going to get from anyone. This pair of wires is dedicated to a single subscriber, you. Cellular phones are radios that connect to the telephone network via over-the-air signals that compete with signals from other cell phones for the same limited radio spectrum. There are different schemes for sharing this spectrum, but eventually, given enough simultaneous calls, some calls will not get through. Radio links are subject to all sorts of variables, including proximity to cell towers, which can result in poor performance or no connection at all. The ability of the central office equipment to determine when your land line phone is "off-hook", illuminate its buttons, or activate its ringer are provided by a DC voltage that originates at the central office. Telephone service provided via optical fiber (which is made of glass and does not conduct electricity) requires access to 120 VAC power at the customer end. A battery-based uninterruptible power supply (UPS) provides power for up to several hours (or longer) in the event of a power outage. Cable television operators provide telephone service via the same electrical coaxial cable that brings TV and/or Internet service to your home. In this case, telephone service is provided by what is known as voice over Internet Protocol (VOIP), in which the voice signals are digitized and sent over the cable in the same manner as any other Internet traffic (and most TV content as well). The availability of VOIP telephone service, whether provided by a telephone company over optical fiber or a cable TV operator via electrical coaxial cable (or, in some cases, optical fiber) is only as good as the availability of the underlying system. In general, if your Internet service is out, your VOIP telephone service will be out.
My personal opinion is that if you can afford to keep your land line, keep it. It is the most reliable telephone service you are going to find, with the possible exception of VOIP over optical fiber from the same telephone company. What matters to customers is service availability: you want it to work whenever you need it. Reliability refers to the average time between failures. Reliable systems have infrequent failures. Mean time to repair is a measure of how long it takes on average to get things working again when failures occur. The combination of high reliability and short repair times produces very high service availability. Land lines that operate over optical fiber may be more reliable than land lines that operate over a pair of metal wires, but the availability of fiber-based systems also depends upon how long the UPS battery lasts during a power outage. Similarly, if all you have is a cell phone and the power is out, you are at the mercy of how long its battery lasts.
If your land line is provided by a pair of metal wires and cost is an issue, ask your telephone company what different levels of service are available to you and what they cost. The most expensive likely will be a package containing features such as unlimited long distance, voicemail, call waiting, call forwarding, and three-way calling. You may be able to get a limited service plan that dispenses with all the other features (that you have already through your cell phone service) and enables you to make local calls. That way you can always make out-going calls when you need to (e.g., 911) even if the power is out and your cell phones are dead.