「上一段・下一段」活用動詞 - why are these groups separate?


My understanding is that the Japanese equivalent of "-iru/eru" verbs or "group
2 verbs" are the groups 上一段活用動詞｛かみいちだんかつようどうし｝ and 下一段活用動詞｛しもいちだんかつようどうし｝,
where 上一段活用動詞 represents the -iru verbs and 下一段活用動詞 represents the -eru verbs.
However, since the conjugations are identical, why is this distinction made?
[This video](https://www.youtube.com/watch?v=PtLaXgMcoRc) seems to make a
distinction between them implying that the differences are as important as the
difference between group 1 and group 3 verbs, but of course this is not the
case. It appears that other grammar resources in Japanese also make an effort
to distinguish 上一段 and 下一段 verbs as well. Why?




> ... grammar resources in Japanese also make an effort to distinguish 上一段 and
> 下一段 verbs as well. Why?

At its most basic, //i// is not //e//. :)

Digging deeper, there do appear to be subtle historical differences. For
instance, many of the 下一段 verbs have corresponding root 五段 verbs, such as つける
↔ つく, or corresponding not-quite-root 五段 verbs, such as ひろめる ↔ ひろまる or でる ↔
だす. Meanwhile, there are much fewer 上一段 verbs to begin with, and these appear
to be less likely to have 五段 near-relatives.

For non-native learners of Japanese, the distinctions between the two are
largely irrelevant: as you note, in terms of the basics of learning
conjugation patterns, the two are mostly identical. You've got to be pretty
deep into Japanese structure and etymology before this starts to matter much.
For native speakers, this is more of a big deal, which may be why monolingual
Japanese materials appear to emphasize this more than English-language
materials do.

