Testnets mirror the rules of the main chain but their coins are free from faucets and carry no value. Protocols use them for rehearsal before launch, and users can practise bridging, swapping or liquidating a position without consequences.
Testnet activity has become a marketing channel, because protocols sometimes reward early testers with an airdrop. That has created professional airdrop-farming which inflates the usage numbers you might otherwise read as demand.
Two warnings: testnet coins are not investments and any offer to buy them is a scam, and testnet performance rarely predicts mainnet behaviour, since the load, fees and adversaries are all different.
Related: airdrop, airdrop-farming, sybil-attack, blockchain