very often are young, to those who are older and more vulnerable and could come down with severe symptoms, or even die (see attached link). Military units have a lot of teenagers and those in their 20s...but they also have senior Non-Comms and Officers who are in their 40s and 50s...they are typically vital and essential to the effectiveness of that fighting force, so having a pandemic overwhelm such forces is a threat, not only to them, but to the people they are trying to protect if they are incapacitated.
Hopefully, this expands your perspective as to why it is important for our military to be vaccinated...the only reason these shots aren't mandatory yet is because they are officially released under "Emergency Authorization"...otherwise it would be just "another vaccine shot" among many they already are required to accept.
Link: https://en.wikipedia.org/wiki/COVID-19_pandemic_on_USS_Theodore_Roosevelt