The existing key lookup (/get/dweet/for/thing-name/key) only checks the single most recent dweet, which returns null if that dweet doesn't happen to contain the key you're looking for — a common problem when multiple devices post independently to the same thing name.
The new endpoint scans back through recent dweets to find the latest value for a given key, regardless of which dweet it was in.
Example — multiple devices posting to the same thing:

curl "https://dweet.cc/dweet/for/thing-name?temperature=21.4&humidity=55"
curl "https://dweet.cc/dweet/for/thing-name?status=ON&power=82.7"

curl https://dweet.cc/get/latest/key/for/thing-name/temperature
"21.4"
curl https://dweet.cc/get/latest/key/for/thing-name/power
"82.7"