It's not going to stop others accessing other apps.
It's useful so you can store photos and other details from prying eyes.
No doubt. Useful if you're organised and disciplined
The original post concerned a feature that allowed security on the hoof, so to speak, and means in an on-the-spot situation, you can hand your phone over to a third party without fear of outing
I take lots of photos, and am often asked to show them to others, with my phone being shared around a few people so they can view. I would rather not have to worry about someone accessing other parts of my phone, either by accident or for mischief.
With me, it's mainly my browsing history. I usually delete my history, but mistakes can (and eventually do) happen.
Many members don't have a separate punting phone, so they could be concerned about publically revealing whatsapp or text messages, social media, or whatever.