Generally:
Mag Plugs are an optional modification that can be fitted to any oil system, on any aircraft. Expensive one's have electric currents that, when closed by the debris on the Plug, can give a cockpit indication. Some posh ones can even send a current to the plug to "burn off" the debris. (known as Fuzz-Burners)
Thay are normally fitted before a filter because the filter catches most of the debris!
Mag Plugs are not there to clean anything. They are there purely as a means of indication for the presence of metallic substances moving around in the oil system. As ST says they won't pick up alluminium particles or rubber seals.
The majority of wear/harm to your engine/gearbox is caused by the ingestion of ferrous metal often originating from within the assembly itself.
Every Mag Plug fitted to your aircraft should be included in your maintenance programme and have periodic inspections. You would do well to keep to those inspections. Such a cheap mod will earn its keep.
If you find anything on one - DONT PULL IT OFF! You should get ALL found debris properly assessed by an expert. At first you may feel this is an unnecessary cost, but it will save you money in the long term. (and if you cant afford it you shouldn't own the aircraft!)
The metallic substance gathered by it may be:
1. Part of the 'build debris' that remains in the system or, more importantly,
2. It may indicate an item beginning to wear or breakdown.
It may show chrome or bright silver coloured particles that may be the finish from bearings or gears that may be classified as "negligible". Sometimes you may see what is described as a "Christmas Tree" - a blob of silver particles all the way up the magnet - Even this (worrying as it looks) MAY be regarded as negligible (by your expert) - depending on what it is and when you get it! It MAY be build debris - It may NOT!
In my experience, the time to worry is when the particles are dull grey or gold.
Hope this helps