Do you need them? That depends on your own personal inclinations and travel habits, and upon a balancing of advantages against disadvantages.
The advantages of advance reservations are obvius. You arrive in a city and immediately check into a hotel without fuss or bother. In some cities, at certain periods of the year, that can save a lot of hotel searching.
The disadvantages are equally obvious. By making advance reservations (which often require deposits), you necessarily must accept a fixed and unalterable schedule for your travels. You cannot, mid-trip, decide to lengten or shorten your stay in a particular city without affecting the reservations you’ve made in other towns. And, of course, you must go through the process of writing ahead to many hotels, some of which may answer that they’re fully booked.
For those who do want the certain of a room at a particular hotel, there are several rules to follow. You must state an exact date of arrival and departure, and not simply an “on or about” estimate. Be prepared to pay an advance deposit on your reservation, to be forfeited if you don’t show up on the exact dates you’ve stated (a hotel may hold your room vacant on that date, and thus lose the chance for other business). You’ll usually find a suitable budget hotel without too much difficulty, dpending on the city and the time of the year.