The first search looks through the Spawn1's room and returns all creeps (yours and others) with a memory.role set to 'Harvester'. The second search looks through all creeps stored in Game.creeps (regardless of if they are in the same room or not, and only returns your creeps) and returns anything with a found memory.role == 'Harvester' similar to before.   Most likely you'll only notice that as soon as a creep leaves your spawn room they will vanish from the first search but not the second.  I haven't messed with filter though so this assumes it returns anything at all.