Many companies have been rethinking the need for offices since the COVID-19 pandemic began, but not MassMutual. The Springfield-based insurer hosted a “topping off” ceremony on Wednesday to celebrate the progress on the 17-story office tower Turner Construction is building for it in Boston’s Seaport District. The event commemorated the last iron beam being placed on the building, which is expected to be completed by the end of 2021. Among those who spoke: CEO Roger Crandall, development partner Joe Fallon, and Mayor Martin J. Walsh. When the 310,000-square-foot tower is done — and MassMutual deems it safe to return to the office in full force — about 1,000 employees will work there. It will be a mix of current employees (about 600 work in Boston today) and new hires. Nearly the entire MassMutual workforce is operating remotely, at least until the first quarter of 2021. Fun coincidence: Turner also built MassMutual’s headquarters in Springfield, back in 1927.