Because I don’t know the maintenance history of the car prior to my ownership (if the previous user varied from 93 to 91 or 87 gas, etc.). And because I felt like it. $3 for the can and 5 minutes of work. It’s a piece of mind thing.
I did it on the Maxima every 30k regardless of whether it needed it or not. Some of you may remember the spreadsheet I had posted when I was selling it. I am very meticulous and sometimes overly obsessive of maintenance. Again, a piece of mind thing. Whether it needed it or not, with my maintenance record (unless I screw with the car), my cars run 100% well into the 6 figure mileage mark. My Maxima, my first Maxima, and the Subaru before them.
I have used it in the crankcase, gas, and IM with good results. I had some sludge on the Maxima and it cleaned that out via the crankcase (I dropped the pan to clean it out and it was surprisingly clean after initially seeing a whole lot of sludge when I had the valve covers off). My fuel system on the Maxima was always in tip top shape and the IM I always see an improvement in general driving (seems to be a little smoother under normal acceleration as if it’s breathing easier). All on par with what many other people have experienced after sea foaming.
My car is an 07, but it already has 50k on it (I’ve put on 18k since I got the car in May).