Using a /31 subnet mask helps conserve your IP address space for point to point links.
It doubles the number of links that you can number from a given block of IP addresses.
If you think of large organisations that have hundreds of WAN links but are rapidly running out of address space.
This is especially useful if you have paid for a block of public addresses from your ISP and so you want to utilize as many of them as possible.
Not only can a /31 subnet be used on serial interfaces but it can also be assigned to ethernet interfaces.