Do I always need RewriteCond? - .htaccess

Question

I have this snippet in my .htaccess file:
RewriteCond %{QUERY_STRING} id=([0-9]+)
RewriteRule ^(20\d\d)/page/([0-9]+)/$ ?yearMeasure=$1&page=$2 [L]
RewriteRule ^(20\d\d)/$ ?yearMeasure=$1 [L]
RewriteRule ^category/([A-Za-z0-9-]+)/page/([0-9]+)/$ ?category=$1&page=$2 [L]
RewriteRule ^category/([A-Za-z0-9-]+)/$ ?category=$1 [L]
RewriteRule ^(20\d\d)/([A-Za-z0-9-]+)/$ ?type=post&year=$1&title=$2 [L,QSA]
However, when I remove the first line, the website still behaves in the same way. The URLs still remain "clean." This brings me to my question; what is the significance of RewriteCond in this snippet?
The only RewriteRule this RewriteCond affected was this:
RewriteRule ^page/([0-9]+)/$ ?page=$1 [L]
...when I attempted to visit /page/2/, it didn't work.
When I moved it above the RewriteCond as follows:
RewriteRule ^page/([0-9]+)/$ ?page=$1 [L]
RewriteCond %{QUERY_STRING} id=([0-9]+)
RewriteRule ^(20\d\d)/page/([0-9]+)/$ ?yearMeasure=$1&page=$2 [L]
RewriteRule ^(20\d\d)/$ ?yearMeasure=$1 [L]
RewriteRule ^category/([A-Za-z0-9-]+)/page/([0-9]+)/$ ?category=$1&page=$2 [L]
RewriteRule ^category/([A-Za-z0-9-]+)/$ ?category=$1 [L]
RewriteRule ^(20\d\d)/([A-Za-z0-9-]+)/$ ?type=post&year=$1&title=$2 [L,QSA]
...everything came back to normal. Why?

Answer 1

In the docs RewriteRule is defined this way:
RewriteRule Pattern Substitution [flags]
If Pattern matches the url requested (domain.com/path/file.cgi), the url is replaced with the Substitution (either a redirect header is returned or the url is changed internally depending on the flags)
This rewrite rule:
RewriteRule ^(20\d\d)/page/([0-9]+)/$ ?yearMeasure=$1&page=$2 [L]
will check if the url matches the pattern and the replacement will occur only in this case. That's why here such RewriteCond is useless:
RewriteCond %{QUERY_STRING} id=([0-9]+)
It's useful in situations that RewriteRule can't capture like checking the hostname or whether a file exists:
RewriteCond %{HTTP_HOST} !^example.com$ [NC]
RewriteRule .? http://example.com%{REQUEST_URI} [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
http://www.sitepoint.com/apache-mod_rewrite-examples-2/