[gentoo-user] IPtables - Mangle table - when/why do I need it (or do I need it)?

[Tanstaafl <tanstaafl@libertytrek.org>]

Hi all,

This has been bugging me for a while...

I've googled, and can't seem to find a definitive answer to this question...

Lots of references to the Mangle table, but nothing that really explains 
what this table is or does, and when or why I would want/need it.

Currently, I have this in my rules (since forever, honestly don't even 
remember where it came from):

*mangle
:PREROUTING ACCEPT [1378800222:449528056411]
:INPUT ACCEPT [1363738727:447358082301]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1221121261:1103241097263]
:POSTROUTING ACCEPT [1221116979:1103240864155]
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG 
FIN,PSH,URG -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
COMMIT
# Completed on Sun Dec 11 14:11:01 2011

This is on a mail/web server with a static IP, it does not do any NAT 
and does not act as a perimeter firewall, it only protects itself...

Thanks for any pointers to tfm that explains this if there is one, or 
just for a simple explanation if not...