You can read about the history of 梵字 here. Briefly, this script is just one of the old scripts used in India, but Japanese Buddhist monks admired it as a sacred script because many Buddhist scriptures imported to Japan were written with it. In those days, India (天竺) meant the promised end of the world. Since the 8th century, 梵字 have been used in Japan as mysterious "symbols" that represent Buddha, Gods and the "truth of the universe".
Today, only experts can read 梵字, but ordinary Japanese people do know they are mysterious characters (or "symbols") related to Buddhism and ancient Asia. Today, 梵字 are used mainly as mysterious design elements with an Asian and/or religious flavor. A T-shirt is indeed one of the places where we see 梵字 often. Asian wizards in fantasy works often use them when casting a spell. If I understand correctly, it's similar to how Western people use alchemical symbols and runic letters as mysterious design elements.