I don’t see Google alter my search query either. Unless you mean the URL like @jaytkay mentions. In which case:
That’s because an URL can’t contain spaces, they are automatically converted to ’%20’ to prevent the space from corrupting the URL. It’s not that uncommon to change those messy ’%20’ parts of the string to something more human readable like ’+’, and in most cases it’s done as part of a safety precaution (filtering out certain characters or combinations and replace them with other characters that can safely be entered into a database query that won’t harm the database itself; otherwise malicious users could enter database queries as their search query and do all kinds of bad things).
Everything after the first question mark of Google’s URL is called the Uniform Resource Identifier (or URI for short), and is a way to visibly/openly exchange data between two separate pages (in this case Google’s start and results page). The results page has some code to decode the URI to what you originally entered as a search query. So why not use a safer method? Because this method allows you to easily copy/paste URL’s and send them to your friends.
So in other words, it doesn’t really change your query, it just encodes it so it can be used for transport, and decodes it once the transport is done.