Not everyone has a yard, and not everyone with a yard can put an antenna in it. Attic and fully indoor antennas are the fallback for apartment dwellers and strict HOA situations, and while they’re never going to outperform an outdoor antenna, they genuinely work well enough to get you on the air.
What an attic actually costs you
An antenna in an attic is completely invisible from outside, which is the whole appeal, but the roofing materials, any metal ductwork, electrical wiring, and the attic’s typically lower height compared to a rooftop mount all reduce performance versus the same antenna installed outdoors. Asphalt shingles with a wood deck cause relatively modest signal loss, while a metal roof can be a serious problem, sometimes shielding the antenna almost entirely depending on the roof’s construction and how the antenna is oriented relative to the metal.
What works reasonably well
A dipole or EFHW bent to fit the available attic space, even zigzagged if a straight run doesn’t fit, remains a solid performer despite the compromises. Magnetic loop antennas, covered elsewhere on this site, are also a genuinely good option for fully indoor operation since their small footprint and high efficiency per unit size make them one of the few antenna types that work reasonably well from inside a living space rather than just an attic.
Managing expectations
You will hear and be heard noticeably worse than an equivalent outdoor antenna, and RFI into household electronics is a more common headache with indoor installs simply due to proximity. But an attic or indoor antenna genuinely gets people on HF, working DX, and enjoying the hobby from situations where an outdoor antenna would be impossible. I’d rather see someone running a compromised indoor antenna and operating regularly than holding off on HF entirely while waiting for a better antenna situation that may never come.
Related reading: Fan Dipoles · The G5RV and Windom · Choosing Feedline Length