Why are streets split into several ways?

Streets are split when one or more properties are different along the street. A non-exhaustive list of such properties would include:

• Speed Limits
• Number of Lanes
• Parking restrictions
• Nature of street lighting
• Bus routes along the street
• One way sections
• No left/right turn restrictions which require a via point
• Changes of jurisdiction (where say, house numbering restarts)
• Change of surface: asphalt to concrete, asphalt to dirt, etc.
• Nature of the source data (some part may have been seen by a mapper, the rest completed from imagery)
• Presence or absence of sidewalks
• etc

Streets are naturally represented as several ways if they are not linear, or if they are dual carriageways.

If a street is split it is best NOT to recombine sections unless using JOSM which forces a check on all mismatched tags. Creating multilinestring topologies representing all parts of a named street should be done outside OSM as a processing step on an extract. For most streets this is trivial, but there are a number of well-understood exceptions which can create problems and for which there are no good widely available solutions.