Windows does a reasonable job of previewing images these days, and you've probably several other image viewers that you can use, if necessary, so installing yet another isn't likely to be high on your list of priorities. And yet, it could be a very good idea. Here's why.
IrfanView isn't just one of those identikit image viewers that handles only the most common file formats, for instance. Along with the usual JPG, PNG, BMP and GIF comes support for viewing no less than 12 digital camera RAW formats, various game formats (WAD, WAL, VTF), fax files (DCX, G3, SFF) and more. The program can view icons directly (ICO), extract them from executables (EXE, DLL), even play some video and audio files.
IrfanView is able to save files in 21 formats, too. A powerful Batch Conversion dialog makes it easy to convert an entire folder structure of images from one format to another.
If you're more interested in working on individual images then you'll appreciate the paint tools, which help you annotate an image by adding text captions, drawing basic shapes, changing colours, and more. There's a reasonable selection of image manipulation tools (flip, rotate, resize, sharpen, colour corrections), and a basic set of special effects (emboss, oil painting, sepia and so on) can be enhanced by downloading additional plugins.
And the more you explore IrfanView, the more smart features you uncover. Like its slideshow feature, that can save a sequence of your favourite images to an EXE or SCR file. The batch scanning option for processing multiple images in a row. The support for editing multi-page TIFFs, or creating multi-page PDF files. And the way you can extend the program even further, by installing free plug-ins from the IrfanView site.