It seems that either I don't understand what these options mean, or bash behaves the same (as interactive shell that reads standard input) without any options anyway. When would I want to use these options explicitly?
From what I can understand it seems to be used for testing purposes. A startup file can use this to test the state as well as a shell script.
By default when you invoke a bash shell, it uses -i and -s so I am assuming for testing purposes you can invoke the shell explicitly with these options via the script or file to test the state that a normal login bash shell would provide.
I found that information from the man page under invocation Esref stated in the comments. It may not be so easily understood though, as it doesnt seem to be something a person would do normally.
A different way to look at Bash invocation
The reference manual has a pretty detailed description on how to invoke Bash. I prefer to document it slightly differently though:
bash <options> <script_file> [<arguments>] bash <options> -c <command_string> [<arguments>] bash <options> [-s [<arguments>]
The first two are non-interactive, and hence don't have the associated features (prompt, job control, command history,...); However, if you need those features, you can enforce them with the -i flag.
The third one is interactive, and the optional -s flag is justified in the paragraphs below...
Example 1: Basic
The -s option can be used to specify arguments:
bash -s a b c
starts a shell... In this shell we can check arguments:
echo $1 $2 $2
reflects the arguments from the bash command:
a b c
Without the '-s' option, the first positional argument would be interpreted as a file name:
bash a b c
bash: a: No such file or directory
Example 2: Using a heredoc
The previous use of the -s option can be used to provide arguments to a heredoc (when you don't want to use a file to embed the commands):
bash -s a b c <<'EOC' echo $1 $2 $3 EOC
a b c
Note that the bash command executes the commands in the heredoc, and then exits; This brings you back to the shell where you invoked the bash command.
Example 3: Pipelining
Consider embedding bash in a pipeline (when you need to implement some inline scripting), again the opportunity to provide arguments can be exploited
echo 'echo HELLO $1' | bash -s WORLD
This is a contrived example, but for some more complex scripting (with a heredoc!), this can come in handy...