"A very quick visual comparison of the setting "bWorkshopEnableGridBasedSnapping" - both OFF (the default, set to "0" in the ini file) and ON (set to "1" in the ini file) - the initialization file is the Fallout4Custom.ini file found in Documents>MyGames>Fallout4.

Changing this setting, alters the game from ‘snapping to other objects’ (eg. a light to a wall, or two wall pieces to each other, as shown here) to ‘snapping to the Settlement grid’ (eg. a light can then be ‘halfway through a wall’ or a flag can be ‘hanging in the air’, looking as though it is “hanging from the rafters of a building”).

Note that there is a small space left, when the "grid based snapping" is off (and the pieces snap to each other).
Note that there is no small space left, after the "grid based snapping" is on (and the pieces are snapping to the grid of the environment).

This can be used/altered without game mods or any DLC (used with "vanilla game" or with addons).

Personally, I utilize the setting off and on when creating a walled-in Settlement; where the pieces do not always snap closely together [and I 'really need it to' at that point in time]. Altering the setting (turning on the Grid Based Snapping) allows the wall pieces to be very close together, overlapping slightly even, allowing for a more complete/fit wall structure. Of course this setting can be used for other Workshop Build items as well (Lights, Decorations, etc).

[Created originally for a Steam Discussion talking about wall pieces snapping together, I hope it helps others too – especially those who like to ‘build up Settlements and forget about the Main Storyline Quests’, like I do! Heh]

HTH
~T

[GIF is resized and compressed to comply with Steam Upload Restrictions]"