As a man, you really should do this at least once on your journey to self-improvement (especially when you’re lifting). I guarantee you have at least one reflective surface in your home where you can see yourself fairly clearly and derive the most benefit from doing this.
Looking at yourself wearing a tank-top (some call it a “wife beater”; I don’t buy into the whole “never hit a girl” bullshit and I’m all for cold-cocking your old lady if you need to defend yourself against a fierce physical assault she herself started, but still) completely increases your confidence as you see your muscle tone increasing and gives you the feeling that you’re invincible and anyone would be moronic to fuck with you. Don’t be stupid about it, though; there will always be someone bigger, faster, or stronger than you so keep that in mind in order to stay out of trouble.
This kind of confidence and feeling can’t be bought or really even put into words so you need to experience it yourself in order to understand the complete effect.
Here’s a decent example of the benefits you can derive. I was at my favorite sushi place this evening while wearing an all-black ensemble consisting of my black tank top, black jeans with chrome chain, and black shoes. I wound up getting eye-fucked by an attractive girl (a 5’3″ slender brunette with immaculate tits, doe eyes, and incredible legs); I’m about 6’5″ and muscular so I generally command dominance when I walk into a room anyway. Still, it was an IoI (Indicator of Interest) but I don’t shit where I eat especially since the (mostly female) staff there all know me. I’d rather maintain my status as a regular than pursue a potential notch (this was still a difficult trade-off to make) but I’m sure you can see what I’m getting at here. You wind up carrying yourself with more confidence and self-worth and appear more attractive to the alternate sex too.