Investors at different points in time have different things to offer a company. An angel investor brings more than money to a company in the first few years (connections, guidance, etc.), but they may not be able to help as the company transitions to a medium-sized firm. That, plus the fact that they still hold a non-negligible share of the company, makes it desirable to cash them out and return control to the core of the company (later investors, executive team, etc.). Especially if they have board seats, it doesn't make sense to have somebody who's not as qualified (or interested) with that much control. So, you buy them out.
I can't speak to how it has been done in the past, but it doesn't make much sense to me to force all involved parties to wait for IPO + 18 months to divest from a company, especially if they're going to actively hinder the companies growth.
Also, the terms you get from shifting pre-existing shares (e.g., employees) around are probably better than creating new ones. The later investors get a greater chunk of the pie for less money because the company doesn't have to create new shares, and the early investors would prefer to get cash today than more highly valued shares tomorrow.
I can't speak to how it has been done in the past, but it doesn't make much sense to me to force all involved parties to wait for IPO + 18 months to divest from a company, especially if they're going to actively hinder the companies growth.
Also, the terms you get from shifting pre-existing shares (e.g., employees) around are probably better than creating new ones. The later investors get a greater chunk of the pie for less money because the company doesn't have to create new shares, and the early investors would prefer to get cash today than more highly valued shares tomorrow.