What is DDS?
DDS is DirectDraw Surface, the texture format games ship. Its point is that the GPU reads the compressed blocks directly, so BC1/BC3 textures never need unpacking at runtime. FileSorcery reads it and converts out; writing good block compression is a job for a dedicated compressor.
What is TIFF?
TIFF is the archival and print format - lossless, high bit depth, and flexible enough to hold multiple pages in one file. Scanners, publishing workflows and document archives lean on it heavily. Browsers have never displayed it, which is why it is offered here as a target rather than a source.