Why and when to use reflow soldering?
Commercially there are two main soldering methods - reflow and wave. &quot;Manual&quot; soldering may still be used to add selected mechanically complex or large parts but this would be rare. &quot;Manual&quot; soldering could include the use of &quot;robots&quot; for the excessively keen.
Wave soldering involves literally passing a wave of molten solder along a carefully preheated board. The board temperature, heating and cooling profiles (non linear), solder temperature, wave shape (even), time in solder, flow rate, board rate and more are all important factors that affect results. Pad shapes and component orientations matter and shadowing of parts by other parts needs to be worked around. All aspects of board design, layout, placement, pad shapes and sizes, heat-sinking and more needs to be carefully considered to get good results.  Where used with SMD components they will need to be retained in position - either with purpose applied adhesive instant set adhesive or advanced magic.
Clearly, wave soldering is an aggressive and demanding process - why use it?
It's used because it is the best and cheapest method when it can be done and the only practical method in some cases. Where through hole components are used wave soldering is usually the method of choice.
So - Reflow soldering is less demanding on pad shape, shadowing, board orientation, temperature profiles (still very important) and more. For surface mount components it is often a very good choice - solder and flux mix are preapplied with a stencil or other automated process, components are placed in position and are often adequately retained by the solder paste. Adhesive may be used in demanding cases. Use with through hole parts is problematic or worse - usally reflow will not be the method of choice for through hole parts.
Where it can be used reflow soldering is used in preference to wave. It is more amenable to small scale manufacture, and generally easier with SMD parts.
Complex and/or high density boards may use a mix of reflow and wave soldering with leaded parts being mounted on one side of the PCB only (call this side A) so they can be wave soldered on side B. Prior to through hole part insertion components can be reflow soldered on side A amidst where TH parts are going to be inserted. Additional SMD parts can then be added to side B to be wave soldered along with the TH parts. Those keen on high-wire acts can try complex mixes with different melting point solders, allowing reflow on side B before or after wave soldering, but that would be very uncommon.
FWIW manual soldering, while slow and expensive, is the least demanding of most factors as it usually also utilises biological computing power to control relatively crude soldering instruments in extremely flexible manners. However, precision of component heating and temperature profiles are poor comparatively.
Some modern components (eg Nichia SMD LEDs with silicone rubber lenses) MUST be reflow soldered  (according to the data sheet) and MUST NOT be hand soldered or wave soldered.