Simply, as the expression implies : the TLM is based on exressing the aspect of a certain system as a set of transactions. For example, if you are modeling a communication network like a WLAN, you don't need to specify any timing for the packets, you don't need to infer any clock .. you can just write thesupposed packets in an input file, read this file from inside the SystemC TX model, and dump the output of the TX send port in another file (say VCD file). Then see the contents of this output file if correctly the TX has modulated the packets and so on.
In RTL, you can't do just this .. you need to specify the clock, reset, enable, registers, .. etc. This is what RTL for.