From what I understand, to eliminate background noise, filter out stations just Khzs apart, etc.
And Elk-- I want something new to start off with. No all around icom rig is cheaper than the Yaesu 897d. That rig also can be operated from batteries and is great for the field. Good forever....
Just wondering if Yaesu might be in the process of designing a new all around rig since the 897 dates back around 10 years...?
The 897 is a fine choice for a first HF rig. Yes, if you want to make it closer to a table-top HF station then you'll want to add at least the SSB filter.
It can operate from an internal battery but only at 30watts output, I believe. If you're going to be camping or doing a lot of portable work then you may want to consider it.
If it is for a base station, then you may want to consider something like the Yaesu FT-950. It's an excellent rig for the price. I think the 450 is a decent radio, but the 950 is pretty good upgrade for not quite double the price.
I own the Icom 7000 and 7600. The 7000 is a great all-in-one radio for HF/VHF/UHF work. It's designed to be mobile and doesn't need any upgrades to work out of the box.
If you're looking to buy just one radio to last you a few years, then I would consider the FT-950 or perhaps the Icom 7600, if the budget allows. Both radios have the roofing filters which will help with weak signal work.
I plan to do all my stuff on CW and Data. Make long distance QSO's a learning and rewarding experience. 
