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Pro Offers a wide range of products
Adafruit offers a large selection of products for all sorts of embedded applications. They provide development boards, programmers, power supply boards, as well as some dedicated breakout boards and actuators.
Pro Products are documented
You'll not have to spend time looking for the datasheets of the individual products since Adarfuit provides full documentation of almost every electronic accessory they sell. In the majority of cases, they even offer product-related Arduino and RaspberryPi libraries as well as some schematics and Gerber files.
Some of the Adafruit's breakout boards are noticeably more expensive than the alternatives from their competitors.