Perhaps consider the question the other way - is there any good reason TO re-use the app ? It's very easy and basically free to create an additional app, and that way you don't run the risk of possibly ending up with settings needed for one scenario that conflict with another scenario's requirements, now or in the future. Here are some other possible considerations though:
new apps require Publisher verification, since 9 Nov 2020. This won't affect you for an internal app, which can be consented to by a global admin.
If you need the user (or admin) consent for some set of privileges (e.g. delegated Graph access), then using the same app might make sense. An example, in a Teams context, might be a bot and a tab that both need to access something from the Graph on the user's behalf. You could get consent in one context, and use it to access the resources from both contexts.
In a nutshell, and especially without a really really good idea of both of your current and planned use cases, it's hard to give a really solid 'yes' or 'no'. My gut says go with a separate app for a separate, unrelated scenario though.
Thanks for the answer. Regarding your point 2. I do require Graph access for my bot to be useful, so it requires a superset of the permissions my existing app is using. OTOH I still want my existing app to be usable without Teams (it's using the "default permissions" admin consent flow and I don't want to mix in bot permissions into that) so it sounds like a separate app ID with a superset of permissions is the way to go.
sounds good. As discussed, there's no cost for an additional app, so it's really just about potential inconvenience for a user to consent twice. If that's not even applicable (same user won't be using both apps), then definitely two apps is better