"Metal Reflect Slime".
All these cards are treated as both Monster Cards and Trap Cards while they are Summoned to the field, and as such, take up both a Monster Card Zone and a Spell & Trap Card Zone at the same time. If they are negated with "Jinzo" or "Royal Decree", they will return to the Spell & Trap Card Zone as Continuous Trap Cards with no effect, and will remain there even after "Jinzo" or "Royal Decree" are removed.
Trap Monsters are typically used as an emergency defense, to stonewall an opponent ("Metal Reflect Slime" is particularly useful in this regard), but they are also used to provide quick Tribute Fodder for higher-level monsters ("Metal Reflect Slime" is great when combined with "Level Eater"), such as the Monarchs, and can in some cases be used offensively as well. Also, because they don't count as monsters while in a player's Deck or Graveyard, some effects might work on them as a Trap Card, while others (like the effect of "Morphing Jar #2") don't see them as a monster. This can be helpful in some cases (see All-Out Attacks OTK).
Another use for Trap Monsters is for Synchro Summoning or Xyz Summoning. Since most Trap Monsters are Level 4, they can be used to quickly Synchro Summon a Level 5-8 monster or Xyz Summon a Rank 4 monster. "Metal Reflect Slime", which is too high a level to be useful for a Synchro Summon, can be used for Rank 10 Monsters such as "Superdreadnought Rail Cannon Gustav Max". "Shape Sister", a Level 2 Tuner, can be used to assist in Synchro Summoning.
"Imperial Custom" makes Continuous Trap Cards indestructible. If you control a copy of "Imperial Custom" and a Trap Monster, the Trap Monster will not be able to be destroyed by battle OR by a card effect. This combination is especially potent in defensive decks.
Trap Monsters may also be useful in a Uria Deck as it will most likely need more monster support and these cards can not only provide it, but can be used as a Tribute to Summon "Uria, Lord of Searing Flames" as well as give him extra ATK points.
If "Book of Moon" is used against an already active Trap Monster, the Trap Monster is set as a Trap Card again and can be activated the next turn.
It is usually a good idea to have "Trap Eater" and "Double Snare" and "Splendid Venus" in the Side Deck of a Deck using high concentrations of these cards, so that you are capable of destroying cards such as "Royal Decree" and "Jinzo" which would cause your Trap Monsters to become useless cards on the field. "Magic Planter" works well if the opponent uses "Jinzo" or another card that negates it and makes it useless. "Magic Planter" allows you to send to the Graveyard a face-up Continuous Trap Card from your side to free up its zone while also allowing you to draw 2 cards.