Most of the standardization work within LTE done initially was emphasizing the Frequency Division Duplex (FDD) mode. In FDD, a paired frequency band is used and uplink and downlink channels are selected from different frequency bands. To be able to use unpaired frequency bands, a time division duplex (TDD) mode is supported as well. In TDD, uplink and downlink in a cell use the same frequency but the transmissions do not happen simultaneously. A major difference as compared to half duplex FDD, is that not only the UEs, but also the eNodeBs cannot receive and transmit at the same time.
The objective is to make LTE a Global standard, so that it can be deployed world-wide. Therefore, unnecessary differences between TDD and FDD are avoided, unless motivated by significant performance benefits.
Due to this, it simplifies implementation of dual mode FDD/TDD equipment. Mainly UEs can be manufactured at a reasonable price which everyone can afford.