With the airlines where you are pre-seated, and which have premium cabins, it ensures that the SLF turn firstly in the right direction and then into the correct aisle.
However with the LoCo's it seems pointless other than to check that right SLF is on right cattle truck. Mistakes do happen specially when a/c are parked close to each other on the tarmac, and I was once bussed to the wrong a/c along with a dozen other pax.
Yesterday I asked an Easyjet FA on boarding why they check the boarding pass, she smiled and said : "Because we're supposed to ....".
They don't always even look properly, not long ago I did a day trip CPT - JNB - CPT and showed the northbound BP which I'd used in the morning, for the southbound flight, I only realised when I saw that 'my' seat was occupied what I'd done, but the FA hadn't realised.