Some Temari stitching techniques and patterns call for using keeper pins... that is, pins that are placed in the mari not so much for indicating where to take a stitch, but for holding threads in place as you work a design or stitch. Without a doubt, keeper pins can be a bane or a blessing; stitchers think they cause more frustration than they help because your thread gets caught on them, others will not work without them were applicable, and others find a happy medium (it's everyone's personal choice - there is no "keeper pin police". If you don't like using them and can achieve the design outcome, that's just fine. You will however, see keeper pins used in many references in temari patterns, so it's worth knowing what they are for). You will usually see keeper pins called for in directions so it's worth knowing what they are needed for.

The most common use  of keeper pins probably is using a pair of pins to hold bundles of threads together when they cross each other (usually at the equator line). Jyouge Douji (continuous north-south stitching) is the most common example.  As you build up rows, a bundle of thread is created at each place they cross each other. It is important to keep them in place, and also to keep them neat (flat and aligned) as you work the design, since later on you will finish them in some way - perhaps either overstitching or wrapping. How neat the appearance of the design turns out will be determined by how well controlled the bundles of threads were as your worked. While it is possible to "groom" the thread alignment after you are through stitching, it usually cannot be done to the degree needed to correct not keeping the threads neat to start with. This will ultimately affect the outcome of your design, no matter how neat your other stitching is. In situations like this the keeper pins are usually kept in place until the design is complete and whatever anchor you are using on them is done.

The other very common use of keeper pins is when working Tsumu Kagari, commonly known as "spindles". Once the spindle shape has been set up after a few rows and the design element is stable, the pins can be removed. This can often happen other times when keeper pins are needed - they may not be needed through the whole project so you can remove them after they serve their purpose.

When it comes to keeper pins being a bigger bane than blessing because of them catching your thread - sometimes it just has to be taken into stride if the design depends on them. However, a few things can help:  Pull your thread a bit more slowly as you complete the stitch so it doesn't twist or tangle, which makes it more likely to catch on the pins. Be picky about your pins: use ones with smooth heads, and perhaps smaller heads. Metal or glass head pins are usually smoother than plastic. Some plastic head pins have "nubs"  - little rough spots left from manufacturing that catch like crazy. You can trim them off or, just be sure when purchasing that you select a pin with a smooth head (it usually depends on the quality and manufacturer). Lastly - when stitching and you are pulling your thread through, turn your mari so the pin heads are upside down - works like a charm to lessen snagging.