Comment by Xylakant

Using a server based session storage is simple as long as your whole app lives in one datacenter and all frontend hosts can reach said server. Once you have app servers in multiple datacenters (e.g. for geo loadbalancing) and want to provide a seamless login no matter which server the user ends up on server based session storage just gets a lot harder. (Apart from having to handle a massive write load which used to cause major pain with mysql, myisam and database based session storage, but these times are luckily over).