This is basic suggestion, some other extra levels give rewards, other than the other foot and excavators. I have no idea what rewards i can suggest that the extra levels can give but i hope the developers can add some.
oh i guess that is true, infact i think the developers are working on this.
This image comes from the August 2019 newsletter, if you haven't seen it. It shows the section "extra level" in achievements of the undergrowth.