Computers eliminate the transcription and entering of data on paper into computers. Large quantities of data can be stored in much smaller spaces. Automatic computation of inter-observer reliabilities are possible. Output files are directly importable into standard statistical and spreadsheet packages. Some event recorders limit your flexibility.
Computers produce a barrier between the observer and the data.
It is as possible to collect too much data as it is to collect too little. Customizing your own software requires time investment with no guarantee of success. That said, there are several kinds of systems on the market now. In a barcode system, you assign codes to barcodes. When you see the behavior, you swipe the barcode.