Merging sequence flows should be modeled explicitly using gateways. This is important for clarity.
Elements (with the exception of gateways) should only have one incoming sequence flow. By modeling more than one incoming sequence flow to an element, the diagram may become harder to read. This is why you should use gateways. It is important to only join split sequence flows with gateways, to avoid making mistakes when modeling. Failure to do so can result in errors such as multi-merges or deadlocks.