not allowed on normal battle/ siege servers
Read the server rules :P
Official sever rules (http://forum.melee.org/announcements/official-server-rules/)
In short you can use a construction site to block a door way in siege. You can also make a weapon rack for range players to reload. However, you are not allow to spam siege equipment including stakes and siege shields
@WarLoard_Lawrence
They are mainly an item for strat but can also be used in normal servers under specific rules (basically no spamming/abusing).
As Hoppster said, you use them with construction material to build certain items.
If you want to practice engineering for strat battles you can buy one and go on EU3/NA3 when its quite and build things (just don't disturb anybody dueling and don't fire the catapult towards people).
Other than that their only use is to build equipment racks for ranged players on siege servers, but their upkeep is quite high so I wouldn't recommend this for a new player.