I'm curious about the structures and procedures that different companies have.
A fairly common arrangement at startups is a purchasing card, which is typically held by a team lead (for larger startups) or the founders/finance folks (for smaller ones). If you need anything, you put it on the p-card. There is generally minimal oversight on "anything" and the limit for a single expense is fairly low ($500 is common, with $1,000 and $250 being probably the next most popular).
If it doesn't fit on a p-card, things get interesting and quite varied per company. For example, a company might have a rule "Devs get to buy their own laptops" with the informal understanding that they're $3k or so, but at the same time not be willing to allow a dev to individually decide to buy e.g. a $3k server. That might be budgeted separately and/or controlled by another process within the company. A dev (or team lead) might have authority to sign off on a one-off $3k expense without elaborate justification required, but it would have to go through purchasing to actually get paid (on a check).
There exist companies which allow employees reasonable discretion to purchase work-related necessities on their personal credit cards and receive reimbursement, typically on a backwards looking monthly basis, after running receipts and/or CC statements past the company bookkeeper. This is very common for sub-$50 purchases like "Bought a pizza for the team because they stayed late" or "Took a prospective hire out to coffee" or "Needed some printing done at Kinkos." Most companies would probably prefer that employees not put SaaS on their own credit cards.
Enterprises are another kettle of fish with regards to processes. In general, lots and lots of scar tissue builds up around this question, and you can pretty much tell who has abused them in the past by what questions they need answered to approve a purchase.
John Sheehan sensibly remarked once that "I need a $25 a month SaaS to do my job. Explain to me the process for getting it. Does that change if it is $250 a month?" is a great question for a candidate to ask when feeling out a company for whether they're desirable to work for or not, since it predicts a whole host of control/micromanagement issues.