TRX is fuel for USDT
For an arbitrage trader the point of TRX is not its own price. The Tron network is the most-used channel for moving USDT between exchanges, and the fee for that transfer is paid in TRX (or through "energy", which is bought with TRX).
So anyone shuttling stablecoins should keep a small TRX balance: without it the transfer simply will not go, and hunting for a couple of dollars' worth of the coin while a route is open is the worst moment to do it.
Trading TRX itself
As a traded asset TRX is liquid and listed almost everywhere. Gaps between exchanges are moderate — this is not TON or a small altcoin. But moving TRX costs pennies and takes seconds, so even a thin percentage can pay for itself.
What to keep in mind
Exchanges set different withdrawal fees for TRX, and on a cheap coin a flat fee weighs more than it appears: one TRX on a coin worth fractions of a dollar is a meaningful slice of a small transfer. Judge the absolute fee against your size, not the percentage.
And do not confuse TRX with USDT on TRC-20: the first is the Tron network's coin, the second is a stablecoin that travels over it. The fee is paid in the former; what you usually move is the latter.