Is HDMI input important on a home theater system?

I am looking to buy the Philips HTS3555 home theater system and i read that it had a HDMI output but no HDMI input. this particular system has a HDDVD player that acts as a reciever. I have a few questions and would appreciate any help!
1. what is a HDMI input for and is it important to have?
2. Basically i want to be able to watch movies in HD with surround sound, will i be able to do that without an HDMI input?

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