There are two different aspects to this question:
Due to the length limitations of 30m of CAT8/Class I & II channels, it is mostly a data center application where the full 100m reach of traditional LAN copper cabling is not needed.
Backbone cabling and cabling for access points could also become an interesting niche application in regular enterprise buildings. Many building backbones are less than 30 meter and many access points under the room ceiling do not need long cabling lengths. It is foreseeable that speeds of access points will in future exceed 10Gbit/s and thus will need better cabling than CAT6A.
Also certain applications for Homes might be a driver for that high speed structured cabling. Since typical length in homes are in the same range as in data centers, “CAT8” might become also a future-proof cabling alternative. Especially the aspect of keeping that cabling for long or ever in a home, the highest available performance is appreciated.
A2: Ethernet speeds:
The most commonly quoted application is 40GBase-T.
There is a relatively newer move in[MK1] IEEE to also create a 25GBase-T ethernet speed.
It turns out that 25GBase-T actually will find more supporters than 40GBase-T.
Why is that?
On one hand cost. Transceivers for 25GBase-T can be produced at lower cost than 40Gbase-T
Secondly, speed aggregation. In data centers, the next higher speed is 100Gbit/s Ethernet, typically on fiber cabling. It is simply easier to aggregate 4 x 25Gbit/s into one 100Gbit/s line that aggregating 40Gbit/s lines into 100Gbit/s lines.