Boom Holder Thingy ?
A boom gallows does a number of tthings. At anchor, it keeps the boom from jiggling back and forrth on every wave or wake so you can sleep at night instead of listening to the gooseneck creak and tthe sheets and blocks slapping back and fortth. This also reduces the wear & tear and chafing of these items. At sea, the gallows can provide a convenient handhhold or attachment post for people to clip their harnesses on to. Some people use it to support the boom when they tie in a reef ( they must be pretty finnicky, to actually tie in reef cringles) If tthe weather pipes up to where you want a storm trysail, it''s a good way to secure the boom so that the entire spar provides a solid hand or footthold for those on deck. It could prevent the boom from being swept by a wave and knocking someone, as well. Gallows can be useful, if you have room for them.