What is the advantage of using git submodules over having a repo inside another repo with a gitignore?

Question

We've been looking into git submodules and we are wondering what advantage (if any) is there in having a repository using submodules verses having a repository within another repository with a .gitignore file.

Example without submodules:

mkdir a
cd a
git init
touch test1.txt
echo "b" > .gitignore
git add .
git commit -m "Adding test1.txt and gitignore"

mkdir b
cd b
git init
touch test2.txt
git add .
git commit -m "Adding test2.txt"

git log
cd ..
git log

Answer 1

The git parent (of the submodules) will keep track of the branches and tag IDs of the submodules when you commit. That will ensure that when you check out the parent (at a known version) then the submodules will also contain their right tags.

If, as above, it just happens to be an ignored subdirectory, then basically these are two independent git repos, as if they'd not been part of a file system hierarchy.