You can split flows into sub-flows for several reasons. Examples: a part of the process is a separate process from a functional perspective, or a part of the process has another owner.
Recommendation: If a process is longer than one lane, consider to split to several flows. Often, in such a long process, you can distinguish one or more sort-of-‘isolated’ parts. So, parts of the process that belong together more than the other parts of the process. For such a logical united part of the process, you can create a sub-flow. Add the sub-flow to the main flow.
Example
The ‘publish to website’ functionality first was fully covered in one flow. However, that became a bit too much and didn’t look that clear anymore. So, now, the process is split into two sub-flows: setup and publish. This is the resulting main flow: