Something I learned during my first job (big box co. tech support and computer sales): self service is great for people who know what they want or know more about what they're buying than the salesperson does. Groceries is a great example of this: I generally know what I like to eat a lot more than most grocery clerks could possibly know, and I get groceries pretty often.

It's less ideal when you don't know much about the product (just that you need one) and don't have the time to go spend a few weeks researching the purchase. I spent some time selling computers: for most customers, they knew what they wanted to do with the computer already (skype a military spouse, work on MS Office Documents, play Skyrim, whatever), but if I was helping them chances were they didn't know how to quantify that in terms of CPU/RAM/HDD/Graphics card/etc., and being able to actually narrow their selection down to a few computers that could meet their needs and explain the differences between them seemed to really put people at ease in what would otherwise been a confusing process.

To expound on my point: I don't know anything about... say... HVAC systems. A self service system for purchasing an HVAC would probably be unnecessarily stressful and confusing for me (at least, it would force me to go become knowledgeable about HVAC systems), and I might even make a mistake and get one that can't actually handle heating and cooling my house. This is an obvious place where a knowledgeable salesperson could save me a lot of time and frustration.