What is the difference between Transformer encoder vs Transformer decoder vs Transformer encoder-decoder?

I know that GPT uses Transformer decoder, BERT uses Transformer encoder, and T5 uses Transformer encoder-decoder. But can someone help me understand why GPT only uses the decoder, BERT only uses encoder, and T5 uses both?

What can you do with just the encoder without decoder, decoder without encoder, and both encoder and decoder?

I'm new to NLP so any help would be nice :D Thanks!