I was wondering why we should pick a single colony for making...

Thanks for the question! There's three main reasons:
1) You want to select colonies that are growing fast and look uniform. Sometimes colonies can be of different sizes reflecting possible mutations in the cell pool. Select the big ones.

2) The whole process happens without antibiotic selection and cultures could get contaminated with airborne bacteria (take into consideration that you are doing this in a lab where the same bacteria is present with plasmids providing antibiotic resistances). You want to select bacterias that can be tracked back to the plate with the streak.

3) E.coli might spontaneously develop spectinomycin resistance. When plated on spec plates, very small colonies will appear. If you use pool of cells, you might end up enriching in these contaminants.