what's the difference between using 'single quotes' or not in find command

The quotes protect the contents from shell wildcard expansion. Run that command (or even simpler just echo *test.txt in a directory with a footest.txt file and then one without any files that end in test.txt and you will see the difference.

$ ls
a  b  c  d  e
$ echo *test.txt
*test.txt
$ touch footest.txt
$ echo *test.txt
footest.txt

The same thing will happen with find.

$ set -x
$ find . -name *test.txt
+ find . -name footest.txt
./footest.txt
$ find . -name '*test.txt'
+ find . -name '*test.txt'
./footest.txt
$ touch bartest.txt
+ touch bartest.txt
$ find . -name *test.txt
+ find . -name bartest.txt footest.txt
find: paths must precede expression
Usage: find [-H] [-L] [-P] [path...] [expression]
$ find . -name '*test.txt'
+ find . -name '*test.txt'
./bartest.txt
./footest.txt