Ads are quite effective for the things you know relatively little about and in general we don't have the time or interest to research most things in our life. For these types of things I often don't even consider alternatives, it's like I have a problem and product X will solve that problem so I buy X and the problem is solved. Now maybe product Y or Z was better or cheaper, but I just reached for the first possible solution. That way I can get on with the rest of my life so I consider this rational even if it's sub-optimal.
What's the first thing that pops into your mind? It's a brand that you've seen a lot, with a memorable catchphrase or jingle. We like to tell ourselves that we don't care about the ads, but the practical reality is that we'd never bother seeking out their store on our own. If they didn't advertise they wouldn't be on the radar. At the grocery store a big poster with an introductory offer can be the difference between "meh, I'll buy the usual" and "okay, I'll give it go... doesn't cost me much to try". Notice the lack of enthusiasm, if I had that I'd buy it anyway. Ads is limping it in.