What is underwater housing for?

The waterproof housing case is essentially a camera case for your Digital camera /Smartphone, but one that allows full control of the camera/ Cellphone when diving, is waterproof and pressure-proof, so the case is designed for Sepecfic camera& Lens models, you should check the compatible list before purchasing.
Matched Camera: for Sony A7R IV

Compatible lenses/ With Standard Flat Port:

Sony FE 90 F2.8 G OSS (Zoom Gear included)

Materials :

Polycarbonate/ ABS plastic/ Stainless steel/ EPDM rubber/ O'ring sealed buttons with spring
Other features:

Sync cable port
High-quality optical viewfinder
Inbuilt leak detection sensor
Fiber optic port
Stainless steel mounting plate 1/4" Thread
Reinforced metallic bottom plate
Multi-coated optical glass lens port

Package list:
1x Underwater housing for Sony A7R IV Black
1x Zoom gear for FE 90 F2.8 G OSS lens
1x User manual/ 1x Spare O'ring/ 1x Shoulder & wrist strap/ 1x Lens protective cover/ 1x Clean cloth