Praying Towards A Sutrah?

I just read some information about having a Sutrah when praying.

If i was praying towards a wall in a room, my prayer mat was mos tly facing towards the wall but one corner of it was facing at my door entrance (and the door was opened), if my mum and other people walked past outside the room, would beSalah be invalidated, as some of my prayer mat wasn't facing towards my wall (Sutrah).

Also, i don't understand, in the Mosque and many other places, no one places a Sutrah infront of them, and people walk infrong people praying, but just not in the area where they are prostrating, but just the row directly infront.

Thank you.