They're also part of affiliate links, as well being used for non-PI source tracking (like search terms), and to specify particular variants when an item listing page contains multiple options. But the basic URL ending with the ASIN will always get one to the item page.
Amazon, as well as other retailers have deeper and more comprehensive ways to track individuals using methods other than URLs, and staying logged in make it even easier.
It really depends on how a site is designed. It may be easier for developers, but personally, as user, I'm not fond of sites that rely heavily on backend machinations and exposed databse inquiries for basic functionality, instead of serving straight pages from simple, easily parsed URLs.
With email links, I'm always cognizant that if I click in a link advertising that "male grooming tools" are on sale, (as a theoretical example), there will shortly be a subseqeunt email highlighting that category, and/or certain products in particular. One site that comes to mind that does this is Crutchfield.
