Body ones can be useful, esp during illness. My mate's ds has very high temp spikes when he gets even a tiny bit poorly, so she gives him calpol and brufen alternately to keep his temp below scary high. She has to keep close eye on degrees creeping up.
I've used them because sometimes when I feel dcs seem hotter or cooler depending on my body temp, so its a handy back up check.
But still, you don't have to have one, also fine without. Just if you wanted to get one, I'd say braun ear thermometer.