Why you end up converting
Different stages of a 3D workflow want different formats. A model designed in Blender starts life as OBJ or GLB; the slicer for your 3D printer needs STL; a laser-engraving toolchain might want PLY or PCD; a website embed wants GLB again. Converting between formats is routine, and most of the time it is a one-click job. A few combinations, though, quietly drop information you might miss.
The common pairings are STL to OBJ (when you want material support on a print-ready model), OBJ to STL (prepping for 3D printing), STL to GLB (web preview or AR), PLY to PCD (handing photogrammetry output to a PCL-based pipeline), and GLB to STL (extracting geometry from a downloaded web model). Each one has a free LaserTools converter that runs entirely in your browser, with no upload, no signup, and no file-size limit beyond your browser's memory.
Browser, desktop, command line
For everyday conversions, a browser tool is the fastest path. The LaserTools converters parse the file locally with Three.js, run the export in the page, and hand you a download. Files never leave your machine, which makes this the right answer for sensitive geometry too. The practical limit is whatever your browser can hold in memory, comfortably hundreds of megabytes on a desktop and less on mobile.
Once files get large, or once you are converting a hundred at a time, desktop software is worth the setup. Blender imports nearly any format and exports OBJ, STL, PLY, glTF, and more through its File menu; it is free and the most universally available. MeshLab is the specialist for mesh processing, with cleanup and simplification options alongside the conversion. CloudCompare is the equivalent for point clouds, reading PLY, PCD, LAS, XYZ, E57, and a long tail of LiDAR formats. FreeCAD covers the CAD side with STEP and IGES alongside the polygonal formats.
For pipelines and batch processing, drop to the command line. Assimp handles forty-plus import formats (assimp export input.stl output.obj). The Python trimesh library is the easiest scripting path for meshes (trimesh.load('input.stl').export('output.obj')), and Open3D is the equivalent for point clouds. The PCL toolchain ships its own converters too, such as pcl_ply2pcd, for projects already inside that ecosystem.
What you lose along the way
Geometry itself almost always survives; the same triangles or points come out the other side. Metadata is where conversions get lossy. Going from OBJ to STL drops materials, textures, UV coordinates, and smooth normals, leaving only the triangles. GLB to STL is similar but worse, since PBR materials, animations, and scene hierarchy all disappear. PLY to PCD keeps the points but may drop custom properties and mesh faces, and it re-encodes colours as packed floats.
The bigger gotcha is mesh to point cloud. A triangle mesh has continuous surfaces; a point cloud has discrete dots. Converting one way does not carry the connectivity, and the other way does not reconstruct it. If your tool needs one and you have the other, conversion alone will not fix it. You need a sampling step (mesh to points) or a triangulation step (points to mesh), and each has its own quality tradeoffs.
Two habits save time. Keep the original and never overwrite your source, and preview the converted file before you trust it downstream. For 3D printing specifically, check for manifold errors after conversion; some formats survive non-watertight geometry, and others silently break it.