It kinda made sense, because there wasn't anything fancy like Notepad. If you wanted to read some section, you had to specify the range. If you wanted to change a line, you overwrote it by using the same line number. If you wanted to insert a line, you had to pick a line number which lies between those other two line numbers. That's why you used an increment of 10 (or whatever) instead of 1.
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It kinda made sense, because there wasn't anything fancy like Notepad. If you wanted to read some section, you had to specify the range. If you wanted to change a line, you overwrote it by using the same line number. If you wanted to insert a line, you had to pick a line number which lies between those other two line numbers. That's why you used an increment of 10 (or whatever) instead of 1.