Ask HN: What benefit is there to requiring sign-in before browsing?

I was just trying to show a student something on aliexpress.com (that I thought would be useful for his project), and the website wouldn't let me browse product categories or search without logging in first.

I suppose LinkedIn does this, as have several other sites that I've noticed.

Why do sites choose this user-hostile approach?

In the case of the student that I was trying to show a part to, I just looked the item up on Amazon instead and Aliexpress lost a sale.

Go figure?