There is a subtle difference between action hooks and filters. The most notable one is that filter functions are expected to receive a value and pass it along in return when done with their work. Inside the function the value can be modified or be used for something else.
In your case the_content is a filter and on invocation it takes a posts content and sends it as first argument to the first hooked function. The hooked function can now modify this content or for example append something. But the function is also expected to finally return the content string, so it can be passed along to the next function hooked to the hook the_content. This is repeated until all hooked functions are done. Then the end product will be returned to the code which called the filter (this filter is called in the_content(), but it can also be invocated in other places) and only then the resulting string is echoed. That is why it is important to always return the string, else the invocating function doesn’t know what to output.
Usage of this filter in you case would look something like this:
add_filter( 'the_content', 'wpse247535_display_news_slider' );
function wpse247535_display_news_slider( $content ) {
if ( is_page( 'sample-page' ) ) {
$content .= "plugin content";
$content .= "more plugin content";
}
return $content;
}
So to answer the title of your question: the content of your page is not overwritten, but your function doesn’t handle the content string it gets as argument and thus doesn’t return it to the invocating function, which then can’t output the content anymore, as it got lost on the way.