There are several reasons the edition of a book might be important to a reader or book collector. I assume almost all book collectors are readers; but not all readers are book collectors.
If you are not reading the book in its original language, different editions may contain completely different translations. The style of the translator can change the reading experience a lot. Often, older translations are looser, and some of them are bowdlerized (they have scandalous/offensive material left out). But some prefer an older style of language to a more modern or contemporary style.
Editing & Extra Materials
Some low-cost reprints of classic works may contain typos or textual errors that were corrected in other publisher’s editions. Publishers with scholarly reputations include copyrighted introductions, endnotes or footnotes, and other material to give the reader more context for understanding the work.
If you are a collector concerned with a book’s publication history, you might want an expensive first edition. If you are a collector concerned with a book’s appearance, you might want a hardcover edition, maybe with a ribbon bookmark and gold edges, maybe even real leather binding. Some people like to get matching sets from the same publisher.
Some books are better made. Rare are the truly clothbound hardcovers with pages stitched in; most books are covered in paper with pages glued together at the spine. Long-lasting books are made of smooth acid-free paper with ink that doesn’t smear off the pages.
If you have difficulty reading small print, you might need to look out for a book that uses a larger font size, wide line-spacing, and generous margins at the page edges. Or get an ebook in a format that allows you to adjust the size and spacing of the text.
The cheapest way to read classics is to download public-domain ebooks from sites like Project Gutenberg and Standard Ebooks. The next-cheapest way is to buy second-hand books from charity or thrift shops and rummage sales or maybe online. The next-cheapest way is to buy new paperbacks from low-cost publishers (like Dover, Wordsworth, and Collins) that reprint public-domain texts.
You get what you pay for?
When you pay more, you typically get more quality in terms of the editing & extra materials, the collectibility, and the physical materials of the book, although of course it’s possible to get a very nice book at a fantastic second-hand price.
You don’t necessarily get better legibility when you pay more: books in the Macmillan Collector’s Library series, for example, are good-looking but they’re small and the text is small. Some larger collector books also have small print. They’re not necessarily meant to be read, they’re just meant to look nice on the shelf.
And as you can see if you browse the translation comparison pages of this site, people have definitely have differing opinions about whether a recent translation is better than a public-domain translation!