Why can't functions such as Log. d be used in other places than onCreate? log. doncreate

Why can't functions such as Log. d be used in other places than onCreate? log. doncreate
I am a self-taught dish. If you have any questions, I 'd like to ask:

Is it necessary to use functions such as Log. d after onCreate?

I used the mainactivity as soon as it came up. I cannot explain the log. d symbol.

I got a reply from the great gods:

Oncreat is a identifier for starting activty. If the program has not started, how can I print logs for you?

When the log10 () function cannot be used in delphi, the following error occurs: Undeclared identifier: 'log'

Directly create a project and put an edit project. The Code is as follows: delphi6 can run normally.
Unit Unit1; Messages, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, Math, StdCtrls; typeTForm1 = class (TForm) Edit1: TEdit; procedure FormCreate (Sender: TObject); private {Private declarations} public {Public declarations} end; varForm1: TForm1; implementation {$ R *. dfm} procedure TForm1.FormCreate (Sender: TObject); begintryedit1.Text: = FloatToStr (Log10 (10); inclutend; end.