Without it the caching of the constructed lexers cannot work (see [nbdev] Lexer
module creates instances of my lexer very often).
The release() method would be useful in case of the lexer caching like this:

public class MyJFlexLexer {
    
    private static MyJFlexLexer cached;
    
    static synchronized MyJFlexLexer create(LexerRestartInfo info) {
        MyJFlexLexer l = cached;
        if (l == null) {
            l = new MyJFlexLexer();
        }
        l.restart(info);
        return l;
    }
    
    void restart(LexerRestartInfo info) {
        // Possibly reuse other classes too and reset them with the given info
    }
    
    public void release() {
        // Possibly reset the structures that could cause memory leaks
        synchronized (MyJFlexLexer.class) {
            cached = this;
        }
    }
    
}