I don't recommend to use it with mains or voltage above 200-300 V. This is not safe probe.
But it can be useful if you're needs to measure 1-5 V signals with low probe capacitance.
That's what I used x100 probes for, back in the day---a cheap substitute for a (then costly,) high impedance, low capacitance, active probe.
It did the job, not perfectly, because the reduced level resulted in a noisy display, but was adequate, & the Boss didn't have to fork out a "King's Ransom" for a rarely used device.
That said, the big old x10 Tek probes as supplied with the 545b were quite adequate for looking at
400-odd volts of ac or DC at relatively modest frequencies, as are the later, smaller ones, so the x100 was a bit of a "rare bird" itself for its supposed intended use.
Of course, the older, analog 'scopes were usually amply rated for input voltage, modern DSOs maybe not so much.