Q.Why do we prefer to express the gain in decibel?

The decibel is a logarithmic unit, which suits both cascaded amplifiers and the way our ears respond.

[!ANSWER]

Because on the decibel (logarithmic) scale the gains of cascaded stages simply add instead of multiply, a very wide range of values is compressed into convenient numbers, and it matches the logarithmic response of the human ear.

[!TLDR]

Decibels turn multiplication into addition and compress a huge range of values.

When several amplifier stages are cascaded, the overall gain is the product of the individual gains, which can be an unwieldy number. Expressed in decibels ($20\log_{10}A_v$ for voltage/current, $10\log_{10}A_p$ for power) the stage gains simply add, making calculation easy. The logarithmic scale also compresses very large and very small ratios into a manageable range and reflects the fact that the human ear perceives loudness logarithmically.

[!ANSWER]

We prefer decibels because the logarithmic scale lets the gains of cascaded stages add rather than multiply, compresses a wide range of gain values into convenient figures, and corresponds to the logarithmic response of the human ear.