Yeah thats it, it's part of the sids guidelines (sudden infant death syndrome) it's thought if you have the baby at the foot of the bed it can't wriggle under blankets and get over heated
My LO slept in a gro bag till she was 8 months old and I didn't always do the feet to foot thing as theres nothing to wriggle down under but then she would get frustrated as she started to learn to roll etc, so now she sleeps with a sheet and a blanket but still not at feet to foot as she wriggles herself to the top of the cot anyway and I end up covering her back up.
But she's 9 months now and old enough to pull covers off, if she was still tiny baby i would use feet to foot