Dreaming that you go to sleep and have another dream can be a device the subconscious mind uses to indicate a change in context or timeframe within the original dream, such as:
A review of an earlier or later situation than that of the original dream
A change to a different level of detail, consciousness, or mood of the dreamer
A portrayal of a "what if" scenario, perhaps that is a bit of a stretch for the dreamer to imagine
A temporary diversion in the dream itself, or representing an actual temporary diversion in the dreamer's life
An aspect of the dreamer's life that the dreamer tends to compartmentalize or to see separately from the rest of their life
A matter that the subconscious mind suddenly decides to explore
A typical shift from one event in a series to the next, then happening to return to the first event
Some other shift in reality or perception