Problem comes when you have a lot of various parts that want to log. Lets say you have a message processing system getting messages from http and throwing that

Problem comes when you have a lot of various parts that want to log. Lets say you have a message processing system getting messages from http and throwing that in a db and keeping counts on an external API. Now every module can use a log library and you, final user of the system can decide where the db log goes, where the http client debug message go, how much detail you want on any of these etc. So in the world of small applications and services, you can come up with a simple logging function. On large projects it is much more complicated.