The monster summoning glyph is a utility glyph that can seem less powerful in comparison to other glyphs because it has have very few immediate benefits. However, using this glyph correctly has many long-term benefits, notably the creation of slowed popcorn that is easy to harvest for strategic leveling.
This glyph is likely to spawn very close to the player, making it ideal for use during early exploration as a means of using mana that would otherwise be wasted. It is also an excellent way to clear low-level chokepoints and reduce the chance of having your hand forced by an unlucky monster placement.
New players may be misled by this glyph into attempting to kill same-level monsters over higher or lower level monsters. This is generally accepted to be an inefficient choice: they grant no bonus experience, but are strong enough to deplete your health and mana. In terms of how much experience reward you get for how much effort you expend, this is a horrible deal and you should constantly be striving to avoid fighting these monsters, either fighting weaker monsters that you can kill effortlessly or stronger monsters that are worth bonus experience.
New players may be further confused by the inclusion of the slow effect on monsters. Most equal-level monsters are too strong to kill in a single blow, and slow will expire as soon as you strike them. This means that you usually cannot capitalize on the +1 experience bonus immediately after the monster is summoned unless you are strong enough to kill it in a single attack. Any character strong enough to do this should be capable of defeating considerably more powerful monsters and earning large amounts of bonus experience for doing so. The slow effect is only venerable in the long term, when you have grown strong enough to dispatch the monster in a single blow.
To use this glyph to its full potential, go to an explored and largely empty part of the dungeon to summon monsters, then leave them there without attacking. This will corral weaker monsters that might otherwise be blocking hallways and place them out of your way, forming what is known as a popcorn bowl. Since slow never expires, you can continue exploring and leveling up and return much later to kill them in a single strike and gain the +1 XP bonus, allowing you to strategically level up at the most opportune time. In many ways, WONAFYT functions as a less expensive but much less controllable and practical version of WEYTWUT.
This glyph is generally only useful during early exploration; hence the tendency to spawn close to the player. During later exploration its mana cost is rarely worthwhile, and as a higher-level character it's unlikely that you will ever be able to slay the monsters it summons in a single blow in order to take advantage of its slow effect (unless you are an Assassin). As a result, if you've already explored more than half the dungeon you should ignore this glyph completely in favour of glyphs with more immediate effects.