Binance claims the move is to "enhance liquidity and capital-efficiency for users", but the conversion and Binance's related decision to stop trading on spot pairs involving those same stablecoins seems like an attempt to increase the status of its own stablecoin against that of rivals.
Binance plans to convert USDC and other stablecoins into their own BUSD stablecoin
Binance users who hold USDC (USD Coin), USDP (Pax Dollar), or TUSD (True USD) will find their holdings "converted" into Binance's stablecoin, BUSD, on September 29. The three stablecoins that Binance plans to convert are the second, fifth, and sixth largest stablecoins on the market as of September 5.