Why is retimer circuit provided for HDMI TX lines?
A retimer is usually used for many reasons.
Usually the reason is that the direct high speed interface pins on some device has incompatible voltage levels or need AC coupling to the next device, so it must be received and retransmitted with voltage levels and DC coupling which HDMI requires to work. The retimer output can also have a stronger drive strength to be able to drive longer cables, and the retiming function can clean any jitter on the input, so the signal quality is higher and it can be better received over longer cables due to that reason too.