What is the use of different ADC channels in ESP32- WROOM32?
The ESP32 has two 12-bit ADCs that can be connected to some 18 pins/channels (plus some that are internal to the ESP32). Simplified, the ADCs use an (analog) multiplexer to select which channels are sampled at any particular time. Actual operation is a bit more complicated.

This allows the ESP32 to monitor many more analog signals than the number of ADCs that are actually available on the chip.

More information can be found in chapter 28 of this document: ESP32 Technical Reference Manual, chapter 28.3