Totally valid question I struggled with for years.
Simply yes, the most of the investment happens when shares are issued - notably the IPO. However companies can issue new shares, or buy back/retire shares and reissue at a future date. The value from people holding shares maintains the price for which companies can cash in at a future date. So demand for shares on the market is sort of a store of value for companies.
Your question on reinvestment: a company is under no obligation to return profits to shareholders and can reinvest as much of their profit as they please. Amazon did this for years. Of course there are practical limits to this as board members often are representatives of shareholder interests. When a company no longer can make new credible investments, they are likely to return excess profits to shareholders. Different companies have different mandates from shareholders and different levels of scrutiny for what counts as credible reinvestment.
Now in practice there is the added layer of shares representing the interests of the make up of the firm. Board seats often are shareholders, creditors give preference to firms with valuable shares, Ceos are often paid in shares as sometimes employees at many firms.
Ultimately, there is no legal claim to profits from shareholders, nor is there legal claim to control. (Look at Google's multi class share obfuscating control). However giving profit and control helps stoke share price which is the store of value for firms as well, helps maintain access to credit/ financial strength, and represents the interests of the firm itself (via its board, creditors, CEO, employees etc.)
TLDR: companies issue /reissue new shares to finance their business. Maintaining share price stores value for business (and new demand builds that value)
Simply yes, the most of the investment happens when shares are issued - notably the IPO. However companies can issue new shares, or buy back/retire shares and reissue at a future date. The value from people holding shares maintains the price for which companies can cash in at a future date. So demand for shares on the market is sort of a store of value for companies.
Your question on reinvestment: a company is under no obligation to return profits to shareholders and can reinvest as much of their profit as they please. Amazon did this for years. Of course there are practical limits to this as board members often are representatives of shareholder interests. When a company no longer can make new credible investments, they are likely to return excess profits to shareholders. Different companies have different mandates from shareholders and different levels of scrutiny for what counts as credible reinvestment.
Now in practice there is the added layer of shares representing the interests of the make up of the firm. Board seats often are shareholders, creditors give preference to firms with valuable shares, Ceos are often paid in shares as sometimes employees at many firms.
Ultimately, there is no legal claim to profits from shareholders, nor is there legal claim to control. (Look at Google's multi class share obfuscating control). However giving profit and control helps stoke share price which is the store of value for firms as well, helps maintain access to credit/ financial strength, and represents the interests of the firm itself (via its board, creditors, CEO, employees etc.)
TLDR: companies issue /reissue new shares to finance their business. Maintaining share price stores value for business (and new demand builds that value)