Historically, issues that might arise when using Zabbix have not been easy to troubleshoot. Not everything that would be useful is always logged. Log level can be increased, but those who still remember the first time they saw what DebugLevel=4 can do will understand why that option usually helped more advanced users. Besides, changing the log level required restart of the daemon, which in many cases would obscure the problem or introduce various other problems like delaying data on proxies in larger installations.
In what could be claimed to be one of the greatest improvements in 2.4 and maybe even beyond that, ability to change log level for a running daemon is coming.