Why are two transistors often used instead of one?
What you have is basically a two stage amplifier - two consecutive amplifiers. In such a circuit configuration the gain of both amplifiers multiply. Since each stage has negative gain in your example, the overall gain is positive again.

So let's say Q1 and R2 have a voltage gain of -10 and Q2 together with R3 create a gain of -10, too. Then the overall gain is 100 which is positive and much larger than the gain of a single stage.

In your example this means the following: If UART_TXD goes High, TX_TTL will go High, too. If you omit Q1 and directly feed Q2 with UART_TXD, then TX_TTL will go Low when UART_TXD is High.