WHY use TPM with bitlocker?

thank you to all, ok i thought that the disk is decrypted BEFORE you see the windows login, not after the windows login (PIN).
You say that a thief cannot enter into windows because it has a pin (generally 4 or 6 digits?) and if he tries to mount the HD on another system it is encrypted with the key on the TPM previous system, OK.

but the windows pin is not so long so it is crackable, and i think it is not a good idea to encrypt a full disk with a key stored into TMP module, but essentially you jus thave to type a 6digit pin to acces the files. ok you COULD use a 10...12...15 digit pin but it makes pointless to use a PIN, it is meant to be an EASIER WAY to access the pc, you you have to remember a long PIN it's better to directly remember the bitlocker password :) :) imho.

Doesn't have to be a PIN. I don't use a PIN. I use a password. Of course you can make the password long and complex.

It would defeat the purpose to decrypt automatically. It only decrypts with PIN, password or bit locker key. It's just that after you sign in you have option to automatically decrypt other drives in the computer without putting in pin/password again

so basically the OS asks the encryption key to the TPM module, only after you successfully pass the windows pin/password. and is that password secure or maybe windows is hackable or bypassable etc? because in my mind i find more secure to press power button, wait a moment and see that the bitlocker password is entered WAY BEFORE the OS is loaded..... because i suppose that at that stage even if the OS has some vulnerabilities at the login page, being in a PRE os phase, any potential attacker could use an "os bug", and the only bud usable would be one about the bitlocker encryption technology, not an OS FAULT.

I know that encryption is really important for privacy, as of late. However, I have never was a fan of M$ bitlocker, I have used TrueCrypt in the past until I found out from WOT and its users that the site and software have been compromised and should not be used. Plus it makes the computer (IMHO) sooo sluggish because it has to read and write encryption OTF. For my really sensitive data, I have used hidden partitions and encryption data vaults which seem to work better and sometimes faster than the whole drive encryption process. Especially with drive data storage units getting bigger evey time you turn around.

Maybe I am wrong, is bitlocker getting faster I run an ASUS Q504UAK 2-in-1 Intel i5 and it is a decent workhorse computer and rugged?

My point? Would it not be better to use hidden partitions and encrypted vaults to hide sensitive data rather than encrypting the whole drive (unless the whole drive is sensitive data, I guess :) ...

I think encryption is important to prevent unautorized access to data, for example when the pc is stolen.
It may be possible that data in "encripted vaults" leaks to unencrypted areas of your system, and full system encryption is more secure in that sense.