What effect does compounding bring when used in the title of a song, poem, story? Sometimes, you see word that are made of two conjoined words, and you wonder what's the point of it. Does it make a difference if the title of your song is "bathwater" instead of "bath water"? There's this song by Britney Spears that's titled "Everytime" instead of "every time". What's the intended effect here, and when should we use it?
Inventing new words, including by compounding, is supposed to be clever, or indicate a new concept that should have its own word, sometimes by linking two words that were previously independent modifiers.
Einstein's theory of General Relativity treated time as another dimension of space, thus making space-time, or spacetime.
Compounds like "no thing" to "nothing" can take on their own meaning, "nothing" is understood in many ways where "no thing" would sound strange. That compound has survived 800 years (I know, nobody ("no body") remembers).
"every time" to "everytime" seems to be a failed effort to make some subtle distinction like this, just like the more successful compounds "peacetime", "overtime", "halftime", "showtime", "timezone".
Inventing new compounds is an attempt to be clever or profound, this can be successful if the compound identifies a unique combination of words to represent a unique concept or condition the public finds useful, or funny, or memorable. That happens, the language is always evolving with new tech, new roles, new kinds of relationships, etc.
Amadeus's answer gives a very good insight in the general use of compounding.
Strictly related to the use of compounding in titles, you may also want to consider rhythm. In your example, "everytime" produces a single beat, opposed to two beats. It thus flows faster and creates the expectation of a dynamic piece. Rhythm is obviously important in music and poetry, however it is not limited to them. Another example is the movie title "lord of war", as opposed to "warlord". Splitting the compound term in its components slows down the beat. It forces you to stop, and in the artificial pause dictated by the separation of words, one starts to think, which is the message of the movie, I guess.