The suffix glossary is useful to a reader who knows the basics of Finnish. Without such knowledge, you may need to try many different ways of analyzing a word and to backtrack to a previous point.
Consider the word jättäisinköhän as an example. If you start from the end and first consider whether n alone could be a suffix here, you need to analyze whether the rest could be a word, possibly recognizing suffixes from it, etc. But if you know Finnish to a sufficient degree, you will first suspect that hän could be a suffix and look it up under -hAn, then consider whether kö (-kO) in the remaining part might be the fairly common interrogative suffix, etc., and you would end up with the analysis jättä|isi|n|kö|hän. Looking up the base word in a dictionary and the meanings of the suffixes in this book, you could then analyze the meaning roughly as “I wonder if I should leave (something somewhere)”. In this example, even the base word jättä- (infinitive jättää) is a derivation, from jäädä with the suffix -ttA, but this is an irregular derivation (due to the shortening of the vowel ää) and difficult to recognize.