I am not really sure what LocalService & NetworkService are for, but...
LocalService is the NT Authority\Local Service user account, the Windows XP
System account.
Obviously, NetworkService has something to do with networks, the
NetworkService account.
The LocalService and NetworkService accounts perform things like synchronize
the time, etc.
Just for more information, Local Service and Network Service accounts
are created for security reasons.
In Windows 2000/NT, system services are launched with "Local System"
credential, which has system-wide privilege as Administrator. So if the
service was attacked, attackers gain the privilege of Local System can
perform system-wide attack.
So Windows XP introduces Local Service and Network Service accounts for
system services. Both run with unprivileged "Limited Users" credential
instead of having full system rights, but Local Service access Windows
network using null sessions, i.e., it uses anonymous credential, while
Network Service access Windows network with the computer account, just
like Local System.
For more information, you may refer to this article:
The Services and Service Accounts Security Planning Guide
http://www.microsoft.com/technet/security/topics/serversecurity/serviceaccount/default.mspx