It is really important to keep door thresholds completely dirt free as dirt can damage the weather stripping and effect the seal against the bottom of the door. It is recommended to hoover them out regularly or as needed and then wipe clean with a wet rag.
Thresholds on sliding doors must be kept as clean as possible as dirt and debris can cause the door to become stiff and difficult to open and close. The threshold for a sliding door must also be lubricated to maintain easy opening and closing. These can be lubricated with WD40.
Door thresholds are often neglected but cleaning them as part of the normal house keeping routine can keep them looking good and save alot of unnecessary time and money.