Best Practice: It is recommended to use single quotes if there are no variables or escape sequences in strings, otherwise, you should use double quotes.
# Literal string (there is no variable inside, it is "text only")
$computerName = 'server01'
# Expanding string (there is a variable inside: $i)
$computerName = "server$i"
Explanation: Single quotes disable the expansion of variables and should be preferred if there is no variable or escape sequence inside strings to reduce unexpected results (ex: if the string contains dollar sign $, etc.).