An in-camera meter sees through your lens, so it can compensate for different focal lengths and filters,
can read your film speed and gives you immediate response upon changing the camera settings, such as the aperture.
Plus it may be able to communicate with your flash for TTL exposures.
But it is reflected light only, often matrix and center weighted, almost never 1 degree spot.
With a hand held meter, you have to make all the adjustments yourself: ISO, shutter speed, aperture, filter factor, etc
but you can approach subjects for incident metering which might give more accurate measuring, you can use spot
metering for the Zone system or just accurate farway measuring, better measure any off the camera strobes and use the
same meter for different cameras.
Now, if you are going to be doing street photography with a 35mm camera, the built-in meter will be better suited, as it
will mean one less thing to carry, one less thing to adjust and you will get fast and good results. Plus with street photography
and BW film, you don't need incident, flash and spot metering.