Strut replacement question: 300CE

Hello all.
I'm about to replace the front struts on my 90 300CE. The repair manual I have makes it look pretty straightforward. Jack the car, use stands,
support the control arm, remove bolts top and bottom, withdraw the unit and reverse the operation to refit.
However, it also call for compressing the spring. Looking at the assembly, I'm not sure why compressing the spring is necessary. Seems like the old strut would simply come out, and a new one goes in. Any comments would be appreciated.
Also, how important is it to clean out the threads on each of the bolt holes, and if so, what would be an appropriate tool in the layman's box.
Many thanks in advance!
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