Yes, if there are restaurants that you know you want to eat at, it is best to make reservations to guarantee that you will be seated. Walk ups are seated only if there is space, but many of the more popular restaurants are fully booked and won't be able to accommodate those without reservations.
Disney's dining reservation system has a known quirk/glitch that makes it harder for odd numbered parties to get a res. You can try 1, then if nothing shows up try putting in a res for 2. Even though a solo diner will be seated at a 2 top, the system wants to fill every seat at the table so it will bypass your request for 1, but if you put in 2, many times you'll get a reservation. Just let the CMs at the podium know you are a solo diner when you check in. We're usually a group of 3 and frequently have to search for a party of 4.
If the reservation required a credit card guarantee, as long as at least one person shows up, you will not be charged for the "missing" person.
There are lounges or bars at some restaurants and they do not require a reservation. Some serve the full menu from the attached restaurant while others have their own menu. Many of the signature restaurants at the resorts, Tutto Gusto in Italy, Rose and Crown Pub (limited menu), '50s Prime Time, Yak& Yeti and most of the TS restaurants in DTD come to mind.