DNA:Protein and Protein:Protein interactions are important for transcription factor function. Note modular structure of transcription factors: one part of the protein is responsible for DNA binding, another for dimer formation, another for transcriptional activation (i.e. interaction with basal transcription machinery).
Dimer formation adds an extra element of complexity and versatility. Mixing and matching of proteins into different heterodimers and homodimers means that three distinct complexes can be formed from two proteins.