It may help give you a clean start. Whether you buy a new sketchbook or not is a personal thing. It can work just to use up blank areas in old ones, or to buy a new one. It's not like they're too expensive or anything - pay attention to what motivates YOU with a question like that. Yeah it's worth it if when you do, it gets you thrilled and gets you moving. But if you buy a new one every time, fill four pages and then stack them up... try getting back into existing ones. Because if you check the dates and pick up an old one you will SEE how much better your new sketches are than the old ones. I do buy art supplies to motivate myself sometimes and it works for me. But others' mileage varies. Usually when I do, that'll get me really going. However, around the middle of them I also get motivated by looking through older ones and going "Wow, I've got more than half the pages left in this and it was a GOOD one..." and then "wow. That is SO much better..." because growth is inevitable. Do whatever works to get yourself into the habit!
By the way, no, I don't watch the number of times I mention brands or actually try to sell any particular brand. I watch for bargain ones. House brands at places like Blick or Jerry's Artarama are also good quality and inexpensive. It is not like I get a sales commission or anything, if I like a brand for something it's just that I like it.