<p>I have seen many apps that take instrument classes and take <code>-javaagent</code> as a param when loading also put a <code>-noverify</code> to the command line.</p>

<p>The Java doc says that <code>-noverify</code> turns off class verification.</p>

<p>However why would anyone want to turn off verification even if they are instrumenting classes?</p>

Start-up time, I'd say. Verification that classes are correct takes some time when the class is loaded. Since classes might be loaded in a lazy fashion (not on app start, but when being used for the first time), this might cause unexpected and undesired runtime delays.
Actually the class does not need to be checked in general. The compiler will not emit any invalid bytecode or class construct. The reason for verification is that the class may be build on one system, get hosted online and is transmitted to you through the unprotected internet. On this path, a malicious attacker might modify the bytecode and create something the compiler might never create; something that can crash the JVM or possibly circumvents security restrictions. Thus the class is verified before it is used. If this is a local application, there is usually no need to check the bytecode again.