An air compressors is a tool, specifically it is a tool to run other tools, unless you just need a volume of air compressed for a SCUBA tank or to inflate a tire. What sort and how big of a compressor you need is going to depend a lot on what sort of tools you need to run with it. A body shop running DA sanders and a paint booth all day long needs a much more robust compressor than an engine shop running impact guns and occasionally a media blast booth.