Condoms are a barrier method: They physically keep people's sexual organs from touching. As a commonly cited (and complained-about) side effect, they can also reduce pleasure and sensation, so it can be tempting to put off putting one on.
Sadly, procrastination rarely works out well for anybody. The longer the rubber stays off, the more time people's bodily fluids have to accidentally tangle up with each other. Studies vary, but somewhere between 17 and 50 percent of people wait even later than the last possible minute, putting a condom on after starting sex [source: Sanders et al.]. That unprotected contact increases the risk of pregnancy and STI transmission.
The right time to put a condom on is before sex, not during.